qemu: process: Ensure that 'beingDestroyed' gets cleared only after VM id is reset

qemu: process: Ensure that 'beingDestroyed' gets cleared only after VM id is reset

Prevent the possibility that a VM could be considered as alive while
inside qemuProcessStop.

A recently fixed bug which unlocked the domain object while inside
qemuProcessStop showed that there's possibility to confuse the state of
the VM to be considered active while 'qemuProcessStop' is processing
shutdown of the VM. Ensure that this doesn't happen by clearing the
'beingDestroyed' flag only after the VM id is cleared.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>