core: Added Exec. Msg. for AddPosixFsStorageDomain

Added an execution message for AddPosixFsStorageDomain.

Change-Id: I1b2bdc2199988785dd37b8b9dcf361fbab387496
Bug-Url: https://bugzilla.redhat.com/1139169
Signed-off-by: Idan Shaby <ishaby@redhat.com>
(cherry picked from commit 4c6e7b684c91e25bdcf877ec74c0d44a51bfb622)