qemu: hotplug: Reset device removal waiting code after vCPU unplug

qemu: hotplug: Reset device removal waiting code after vCPU unplug

If the delivery of the DEVICE_DELETED event for the vCPU being deleted
would time out, the code would not call 'qemuDomainResetDeviceRemoval'.

Since the waiting thread did not unregister itself prior to stopping the
waiting the monitor code would try to wake it up instead of dispatching
it to the event worker. As a result the unplug process would not be
completed and the definition would not be updated.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1428893
          https://bugzilla.redhat.com/show_bug.cgi?id=1427801