mount: Add PB entries for FS types

mnt_entry::fstype is a part of image ABI, thus we need
to provide some "common" encoding outside tools would
know about this field encoding.

Thus we instorduce fstype enum in .proto file and use it
in source code as well.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>