sd: improve metadata sizes definition

This refactoring is moving the definition of the metadata sizes in a
dictionary using integers instead of strings.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1083476
Change-Id: I0fe1381ed0883194e283e457e3c948159c68c01d
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26412
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26987