If there are two concurrent threads, one of which is removing a
domain from the list and the other is trying to add it back they
may serialize in the following order:

1) vm->removing is set and @vm is unlocked.
2) The tread that's trying to add the domain onto the list locks
   the list and tries to find, if the domain already exists.
3) Our lookup functions say it doesn't, so the thread proceeds to
   virHashAddEntry() which fails with 'Duplicate key' error.

This is obviously not helpful error message at all.

The problem lies in our lookup functions
(virDomainObjListFindByUUIDLocked() and
virDomainObjListFindByNameLocked()) which return NULL even if the
object is still on the list. They do this so that the object is
not mistakenly looked up by some driver. The fix consists of
moving 'removing' check one level up and thus allowing
virDomainObjListAddLocked() to produce meaningful error message.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>