Check for domain liveness in qemuDomainObjExitMonitor

Check for domain liveness in qemuDomainObjExitMonitor

The domain might disappear during the time in monitor when
the virDomainObjPtr is unlocked, so the caller needs to check
if it's still alive.

Since most of the callers are going to need it, put the
check inside qemuDomainObjExitMonitor and return -1 if
the domain died in the meantime.