Real secondary clients:
 currently the "main" is the client that connects when there are no clients. Main
 should be one of the clients, determined by any means - seniority is fine.

SPICE_MSG_MAIN_CHANNEL_IN_USE
SPICE_MSG_MAIN_CHANNEL_AVAILABLE
SPICE_MSG_MAIN_CHANNEL_NEW
 - allow channels to become available or not. Useful for USB redirection
 and later chardev remoting in the case where there chardev/usbredir channel is
 single user only, so only one of the clients can use it.

Chardev channel: allow adding a chardev channel without any server changes:
 qemu -spicevmc name=generic,channelname=bla,id=bla -mon chardev=bla
 Note: client will still need to decide what to do with it of course. So the channelname
 will have to have a predefined meaning.