core: block migration for paused vm only in case of an error

We used to block migration for every VM which is in paused state but
actually VMs which are in paused state not due to an error should
be able to migrate so we should not block it for such VMs.

Change-Id: Ic6e7a7d41fcf66e2d3c13b93de7dd68b42aeaecb
Signed-off-by: Arik Hadas <ahadas@redhat.com>