qga: Move Linux-specific FS freeze/thaw code to a separate file

In the next patches we are going to add FreeBSD support for QEMU Guest
Agent. In the result, code in commands-posix.c will be too cumbersome.

Move Linux-specific FS freeze/thaw code to a separate file commands-linux.c
keeping common POSIX code in commands-posix.c.

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>