um: vfio: Support adding devices via mconsole

It can be used when we want to pass through PCI devices to UML
while it's up and running. PCI devices can be passed through to
UML using the same syntax as the command line option:

(mconsole) config vfio_uml.device=<domain:bus:slot.function>

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250709010021.1076902-1-tiwei.bie@linux.dev


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