ivshmem: fix memory_region_del_eventfd assertion failure

We do not register ioeventfds unless the IVSHMEM_IOEVENTFD feature
is set.  The same feature must be checked before releasing the eventfds.
Regression introduced by commit 563027c (ivshmem: use EventNotifier and
memory API, 2012-07-05).

Reported-by: Cam Macdonnell <cam@cs.ualberta.ca>
Tested-by: Cam Macdonnell <cam@cs.ualberta.ca>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
