qemu_migration: Check for active domain after talking to remote daemon

qemu_migration: Check for active domain after talking to remote daemon

Once we called qemuDomainObjEnterRemote to talk to the destination
daemon during a peer to peer migration, the vm lock is released and we
only hold an async job. If the source domain dies at this point the
monitor EOF callback is allowed to do its job and (among other things)
clear all private data irrelevant for stopped domain. Thus when we call
qemuDomainObjExitRemote, the domain may already be gone and we should
avoid touching runtime private data (such as current job info).

In other words after acquiring the lock in qemuDomainObjExitRemote, we
need to check the domain is still alive. Unless we're doing offline
migration.

https://bugzilla.redhat.com/show_bug.cgi?id=1589730

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>