qemu: Don't report failure to destroy a destroyed domain

qemu: Don't report failure to destroy a destroyed domain

When destroying a domain libvirt marks it internally with a
beingDestroyed flag to make sure the qemuDomainDestroyFlags API itself
cleans up after the domain rather than letting an uninformed EOF handler
do it. However, when the domain is being started at the moment libvirt
was asked to destroy it, only the starting thread can properly clean up
after the domain and thus it ignores the beingDestroyed flag. Once
qemuDomainDestroyFlags finally gets a job, the domain may not be running
anymore, which should not be reported as an error if the domain has been
starting up.

https://bugzilla.redhat.com/show_bug.cgi?id=1445600

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>