qga: Add UFS freeze/thaw support for FreeBSD

UFS supports FS freezing through ioctl UFSSUSPEND on /dev/ufssuspend.
Frozen FS can be thawed by closing /dev/ufssuspend file descriptior.

Use getmntinfo to get a list of mounted FS.

Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>