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

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>