qga: minimal support for fstrim for Windows guests

Unfortunately, there is no public Windows API to start trimming the
filesystem. The only viable way here is to call 'defrag.exe /L' for
each volume.

This is working since Win8 and Win2k12.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
CC: Michael Roth <mdroth@linux.vnet.ibm.com>
CC: Stefan Weil <sw@weilnetz.de>
CC: Marc-André Lureau <marcandre.lureau@gmail.com>
* check g_utf16_to_utf8() return value for GError handling instead
  of GError directly (Marc-André)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
