Fix race condition when destroying guests

Fix race condition when destroying guests

When running virDomainDestroy, we need to make sure that no other
background thread cleans up the domain while we're doing our work.
This can happen if we release the domain object while in the
middle of work, because the monitor might detect EOF in this window.
For this reason we have a 'beingDestroyed' flag to stop the monitor
from doing its normal cleanup. Unfortunately this flag was only
being used to protect qemuDomainBeginJob, and not qemuProcessKill

This left open a race condition where either libvirtd could crash,
or alternatively report bogus error messages about the domain already
having been destroyed to the caller

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 81621f3e6e45e8681cc18ae49404736a0e772a11)

Conflicts:

  src/qemu/qemu_driver.c - virReportError had been removed from
      upstream in cases where qemuProcessKill failed, creating
      different context.