virttest: fix save_to_file time when use qmp monitor

when QMP monitor used, migrate complete check always failed until
timeout; This commit update check method and raise QemuMigrationTimeout
expection when migrate timeout;
Signed-off-by: NXu Tian <xutian@redhat.com>

tests/cfg/boot_savevm.cfg

@@ -14,6 +14,7 @@
         - default_savevm:
 
         - with_floppy:
+            no Host_RHEL.7
             virt_test_type = qemu
             save_method = save_to_tag
             with_floppy = yes