qga-win: don't hang if vss hold writes timeout

When VM is in a heavy IO, if the command "guest-fsfreeze-freeze"
is executed, VSS may timeout when trying to hold writes.

Inside guest, Event ID 12298(VSS_ERROR_HOLD_WRITES_TIMEOUT)
is logged in the Event Viewer.

At that time, if we call AbortBackup, qga may hang forever.

This patch will solve this issue.

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: Tomoki Sekiyama <tomoki.sekiyama@gmail.com>

Signed-off-by: Chen Hanxiao <chenhanxiao@gmail.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/vss-win32/requester.cpp

@@ -419,6 +419,16 @@ void requester_freeze(int *num_vols, ErrorSet *errset)
             break;
         }
     }
+
+    if (wait_status == WAIT_TIMEOUT) {
+        err_set(errset, E_FAIL,
+                "timeout when try to receive Frozen event from VSS provider");
+        /* If we are here, VSS had timeout.
+         * Don't call AbortBackup, just return directly.
+         */
+        goto out1;
+    }
+
     if (wait_status != WAIT_OBJECT_0) {
         err_set(errset, E_FAIL,
                 "couldn't receive Frozen event from VSS provider");
@@ -432,6 +442,8 @@ void requester_freeze(int *num_vols, ErrorSet *errset)
     if (vss_ctx.pVssbc) {
         vss_ctx.pVssbc->AbortBackup();
     }
+
+out1:
     requester_cleanup();
     CoUninitialize();
 }