domain: use shared lock for extendStorageDomain

The extendStorageDomain command shouldn't hold an exclusive lock on
the storage domain since it can be executed also during other long
tasks (e.g.: moveImage).

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=876663
Change-Id: I2ea73b76b4db010a6eacd80c8328afe66e7cdc51
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10451
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>