qemu: Avoid calling qemuProcessStop without a job

qemu: Avoid calling qemuProcessStop without a job

Calling qemuProcessStop without a job opens a way to race conditions
with qemuDomainObjExitMonitor called in another thread. A real world
example of such a race condition:

  - migration thread (A) calls qemuMigrationWaitForSpice
  - another thread (B) starts processing qemuDomainAbortJob API
  - thread B signals thread A via qemuDomainObjAbortAsyncJob
  - thread B enters monitor (qemuDomainObjEnterMonitor)
  - thread B calls qemuMonitorSend
  - thread A awakens and calls qemuProcessStop
  - thread A calls qemuMonitorClose and sets priv->mon to NULL
  - thread B calls qemuDomainObjExitMonitor with priv->mon == NULL
  => monitor stays ref'ed and locked

Depending on how lucky we are, the race may result in a memory leak or
it can even deadlock libvirtd's event loop if it tries to lock the
monitor to process an event received before qemuMonitorClose was called.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>