KVM: PPC: Add dequeue for external on BookE

Commit a0abee86af2d1f048dbe99d2bcc4a2cefe685617 introduced unsetting of the
IRQ line from userspace. This added a new core specific callback that I
apparently forgot to add for BookE.

So let's add the callback for BookE as well, making it build again.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>