core: Adding exclusive lock (by name) to ImportVmCommand

In order to solve the above bug, an exclusive lock by name
is acquired. The lock is removed after a vm is added to the DB.

Change-Id: I298f7f277a2000537f0edcdd0fcfd8aedd8037d1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=907132
signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>