core: add the domain detaching state

This patch adds an additional "Detaching" domain state used to track
the ongoing operation so that we can do more informed decisions (for
example building the storage pool domains map).

Change-Id: I5732ef00a67ef1381ee0b6f29d08ab39cf63a1bf
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>