mounts: Optimize mount loop device resolution

Existing code was re-reading mtab for each /proc/mounts entry.
Added lookup dictionary that translates /dev/loop spec to actual file
spec.

Lookup is reinitialized each time mtab is changed.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1112779
Change-Id: I54f11786b45782cedd994d52e1e506292132fa47
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28586
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Tested-by: Saggi Mizrahi <smizrahi@redhat.com>