core: fix canDoAction check in RemoveStoragePoolCommand

After change 60cbca3 (core: Detach storage domain when remove Storage
Pool) trying to remove a datacenter without any storage domain resulted
in a IndexOutOfBoundsException.

Change-Id: I209fea6f4a8c9da74a3725ef539442446914de9c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>