Add support for re-exec() of virtlockd upon SIGUSR1

Add support for re-exec() of virtlockd upon SIGUSR1

The virtlockd daemon maintains file locks on behalf of libvirtd
and any VMs it is running. These file locks must be held for as
long as any VM is running. If virtlockd itself ever quits, then
it is expected that a node would be fenced/rebooted. Thus to
allow for software upgrads on live systemd, virtlockd needs the
ability to re-exec() itself.

Upon receipt of SIGUSR1, virtlockd will save its current live
state out to a file /var/run/virtlockd-restart-exec.json
It then re-exec()'s itself with exactly the same argv as it
originally had, and loads the state file, reconstructing any
objects as appropriate.

The state file contains information about all locks held and
all network services and clients currently active. An example
state document is

 {
    "server": {
        "min_workers": 1,
        "max_workers": 20,
        "priority_workers": 0,
        "max_clients": 20,
        "keepaliveInterval": 4294967295,
        "keepaliveCount": 0,
        "keepaliveRequired": false,
        "services": [
            {
                "auth": 0,
                "readonly": false,
                "nrequests_client_max": 1,
                "socks": [
                    {
                        "fd": 6,
                        "errfd": -1,
                        "pid": 0,
                        "isClient": false
                    }
                ]
            }
        ],
        "clients": [
            {
                "auth": 0,
                "readonly": false,
                "nrequests_max": 1,
                "sock": {
                    "fd": 9,
                    "errfd": -1,
                    "pid": 0,
                    "isClient": true
                },
                "privateData": {
                    "restricted": true,
                    "ownerPid": 1722,
                    "ownerId": 6,
                    "ownerName": "f18x86_64",
                    "ownerUUID": "97586ba9-df27-9459-c806-f016c8bbd224"
                }
            },
            {
                "auth": 0,
                "readonly": false,
                "nrequests_max": 1,
                "sock": {
                    "fd": 10,
                    "errfd": -1,
                    "pid": 0,
                    "isClient": true
                },
                "privateData": {
                    "restricted": true,
                    "ownerPid": 1784,
                    "ownerId": 7,
                    "ownerName": "f16x86_64",
                    "ownerUUID": "7b8e5e42-b875-61e9-b981-91ad8fa46979"
                }
            }
        ]
    },
    "defaultLockspace": {
        "resources": [
            {
                "name": "/var/lib/libvirt/images/f16x86_64.raw",
                "path": "/var/lib/libvirt/images/f16x86_64.raw",
                "fd": 14,
                "lockHeld": true,
                "flags": 0,
                "owners": [
                    1784
                ]
            },
            {
                "name": "/var/lib/libvirt/images/shared.img",
                "path": "/var/lib/libvirt/images/shared.img",
                "fd": 12,
                "lockHeld": true,
                "flags": 1,
                "owners": [
                    1722,
                    1784
                ]
            },
            {
                "name": "/var/lib/libvirt/images/f18x86_64.img",
                "path": "/var/lib/libvirt/images/f18x86_64.img",
                "fd": 11,
                "lockHeld": true,
                "flags": 0,
                "owners": [
                    1722
                ]
            }
        ]
    },
    "lockspaces": [

    ],
    "magic": "30199"
 }

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>