qemu: migration: use consistent error message

The other two DomainHasBlockJob usage error messages don't contain
'an', so unify things to save translators some effort. Dropping
the 'an' is closer to the sentence structure in the errors from
qemuDomainDiskBlockJobIsActive as well

src/qemu/qemu_migration.c

@@ -2301,7 +2301,7 @@ qemuMigrationIsAllowed(virQEMUDriverPtr driver,
 
         if (qemuDomainHasBlockjob(vm, false)) {
             virReportError(VIR_ERR_OPERATION_INVALID, "%s",
-                           _("domain has an active block job"));
+                           _("domain has active block job"));
             return false;
         }