um: virtio_uml: send SET_MEM_TABLE message with the exact size

The rust based userspace vhost devices are very strict and will not
accept the message if it is longer than required. So, only include the
data for the first memory region.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241103212854.1436046-2-benjamin@sipsolutions.net


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