um: virtio: allow devices to be configured for wakeup

With all the IRQ machinery being in place, we can allow virtio
devices to additionally be configured as wakeup sources, in
which case basically any interrupt from them wakes us up. Note
that this requires a call FD because the VQs are all disabled.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>