qemuMigrationSrcRun: Re-check whether VM is active before accessing job data

qemuMigrationSrcRun: Re-check whether VM is active before accessing job data

'qemuProcessStop()' clears the 'current' job data. While the code under
the 'error' label in 'qemuMigrationSrcRun()' does check that the VM is
active before accessing the job, it also invokes multiple helper
functions to clean up the migration including
'qemuMigrationSrcNBDCopyCancel()' which calls 'qemuDomainObjWait()'
invalidating the result of the liveness check as it unlocks the VM.

Duplicate the liveness check and explain why. The rest of the code e.g.
accessing the monitor is safe as 'qemuDomainEnterMonitorAsync()'
performs a liveness check. The cleanup path just ignores the return
values of those functions.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>