Some, but not all, codepaths in the qemuMonitorOpen() method
would trigger the destroy callback. The caller does not expect
this to be invoked if construction fails, only during normal
release of the monitor. This resulted in a possible double-unref
of the virDomainObjPtr, because the caller explicitly unrefs
the virDomainObjPtr  if qemuMonitorOpen() fails

* src/qemu/qemu_monitor.c: Don't invoke destroy callback from
  qemuMonitorOpen() failure paths