um: virtio_uml: fix call_fd IRQ allocation

If the device does not support slave requests, then the IRQ will not yet
be allocated. So initialize the IRQ to UM_IRQ_ALLOC so that it will be
allocated if none has been assigned yet and store it slightly later when
we know that it will not be immediately unregistered again.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241103212854.1436046-4-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>