core: reset CPU profile when changing VM cluster

Since CPU profile is coupled to cluster, when changing a cluster
the 'old' CPU profile is invalid.
The edit VM command will validate a right profile for
new cluster.

Change-Id: I10ce4aa64ffddf500f11c89bb942d9abbac8cf31
Bug-Url: https://bugzilla.redhat.com/1084930
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>
(cherry picked from commit c9401694b4471e0b30db5d7eba2f9568f0818847)