Snapshot failure due to fsfreeze with Windows guest

Issue

• During a live snapshot of a Windows VM with an active guest agent, the fsfreeze operation hung, resulting in subsequent libvirt-related errors being reported by VDSM for that VM;

cannot acquire state change lock (held by remoteDispatchDomainFSFreeze)

-The WIndows guest was unable to sync its file systems.

• The VM became unresponsive;

jsonrpc.Executor/3::WARNING::2018-04-09 15:01:15,730::vm::4903::virt.vm::(_setUnresponsiveIfTimeout) vmId=`147d2e3d-709a-4eed-af01-9a776370aadf`::monitor became unresponsive (command timeout, age=99492.55)