qemu: fix race in signal interrupt during QEMU startup

qemu: fix race in signal interrupt during QEMU startup

If a Ctrl-C arrives while we are in the middle of executing the
virDomainCreateXML call, we will have no "virDomainPtr" object
available, but QEMU may none the less be running.

This means we'll never try to stop the QEMU process before we
honour the Ctrl-C and exit.

To deal with this race we need to postpone quit of the event
loop if it is requested while in the middle of domain startup.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>