xen: implement support for secondary consoles in the console backend

This patch corresponds to commit
840184a106bc24e745beda5c77e392f6cecd2bc9 from
git://xenbits.xensource.com/qemu-xen-unstable.git

.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>