um: virtio: Use dynamic IRQ allocation

This separates the devices, which is better for debug and for
later suspend/resume and wakeup support, since there we'll
have to separate which IRQs can wake up the system and which
cannot.

Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>