KVM: Device Assignment: Free device structures if IRQ allocation fails

When an IRQ allocation fails, we free up the device structures and
disable the device so that we can unregister the device in the
userspace and not expose it to the guest at all.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>