fileSD: enable mailbox on file domains

Since we now support mixed-type storage pools (block and file) we
must enable the mailbox on file domains in order to satisfy the
extension requests needed by block domains.
On file domains the inbox and outbox files were prepared empty so
we also need to extend them to the proper size as soon as possible:

* on SPM start
* on master migration

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1083476
Change-Id: Iba8036e33ed8718d05e9d83cad1d63949794f067
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26414
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>