Before issuing monitor commands it is neccessary to check whether
the guest is still running. Most places use virDomainIsActive()
correctly, but a few relied on 'priv->mon != NULL'. In theory
these should be equivalent, but the release of the last reference
count on priv->mon can be delayed a small amount of time until
the event handler is finally deregistered. A further ref counting
bug also means that priv->mon might be never released. In such a
case, code could mistakenly issue a monitor command and wait for
a response that will never arrive, effectively leaving the QEMU
driver waiting on virCondWait() forever..

To protect against these possibilities, make sure all code uses
virDomainIsActive(), not 'priv->mon != NULL'

* src/qemu/qemu_driver.c: Replace 'priv->mon != NULL' with
  calls to 'priv->mon != NULL'()