qemu: Clear block copy mirror state explicitly

While this should not be necessary as we clear it in the event handler,
let's be sure and clear it prior to starting the job.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_driver.c

@@ -17847,6 +17847,8 @@ qemuDomainBlockCopyCommon(virDomainObjPtr vm,
     if (!(job = qemuBlockJobDiskNew(disk, QEMU_BLOCKJOB_TYPE_COPY, device)))
         goto endjob;
 
+    disk->mirrorState = VIR_DOMAIN_DISK_MIRROR_STATE_NONE;
+
     /* Actually start the mirroring */
     qemuDomainObjEnterMonitor(driver, vm);
     /* qemuMonitorDriveMirror needs to honor the REUSE_EXT flag as specified