core: allow to perform hotplug/unplug/attach when vm is paused.

When a VM is paused there's no reason not to allow hot
plugging/unplugging/attaching a disk.

Change-Id: I6d950a1e7945e43ec5ee5ffa0f79c5f9536473a6
Bug-Url: https://bugzilla.redhat.com/918498
Signed-off-by: Liron Aravot <laravot@redhat.com>