hsm: Fix missing MAX_DOMAINS constant

Commit ab6014b394 moved MAX_DOMAINS from sp to spbackends without
updating all users, breaking creation of a pool on block storage using
old storage domain format. This patch fix the error by importing the
name from its new module.

Change-Id: I772b01be4de309aa4b15bf2c89e69b64787e1a11
Bug-Url: https://bugzilla.redhat.com/1059108
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24242
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>