qemu: support type=hostdev network device live hotplug attach/detach

qemu: support type=hostdev network device live hotplug attach/detach

qemuDomainAttachNetDevice

  - re-ordered some things at start of function because
    networkAllocateActualDevice should always be run and a slot
    in def->nets always allocated, but host_net_add isn't needed
    if the actual type is hostdev.

  - if actual type is hostdev, defer to
    qemuDomainAttachHostDevice (which will reach up to the NetDef
    for things like MAC address when necessary). After return
    from qemuDomainAttachHostDevice, slip directly to cleanup,
    since the rest of the function is specific to emulated net
    devices.

  - put assignment of new NetDef into expanded def->nets down
    below cleanup: (but only on success) since it is also needed
    for emulated and hostdev net devices.

qemuDomainDetachHostDevice

  - after locating the exact device to detach, check if it's a
    network device and, if so, use toplevel
    qemuDomainDetachNetDevice instead so that the def->nets list
    is properly updated, and 'actual device' properly returned to
    network pool if appropriate. Otherwise, for normal hostdevs,
    call the lower level qemuDomainDetachThisDevice.

qemuDomainDetachNetDevice

  - This is where it gets a bit tricky. After locating the device
    on the def->nets list, if the network device type == hostdev,
    call the *lower level* qemuDomainDetachThisDevice (which will
    reach back up to the parent net device for MAC address /
    virtualport when appropriate, then clear the device out of
    def->hostdevs) before skipping past all the emulated
    net-device-specific code to cleanup:, where the network
    device is removed from def->nets, and the network device
    object is freed.

In short, any time a hostdev-type network device is detached, we must
go through the toplevel virDomaineDetachNetDevice function first and
last, to make sure 1) the def->nnets list is properly managed, and 2)
any device allocated with networkAllocateActualDevice is properly
freed. At the same time, in the middle we need to go through the
lower-level vidDomainDetach*This*HostDevice to be sure that 1) the
def->hostdevs list is properly managed, 2) the PCI device is properly
detached from the guest and reattached to the host (if appropriate),
and 3) any higher level teardown is called at the appropriate time, by
reaching back up to the NetDef config (part (3) will be covered in a
separate patch).