qga-win: add support for qmp_guest_fsfreeze_freeze_list
This patch add support for freeze specified fs.
The valid mountpoints list member are [1]:
The path of a mounted folder, for example, Y:\MountX\
A drive letter, for example, D:\
A volume GUID path of the form \\?\Volume{GUID}\,
where GUID identifies the volume
A UNC path that specifies a remote file share,
for example, \\Clusterx\Share1\