Advertise MULTI_CONN on writeable NBD servers
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1708300
Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=45354453
Upstream Status: YES
Advertise the NBD_FLAG_CAN_MULTI_CONN for all qemu NBD server situations that support multiple clients, not just read-only.