pv-qemu 10/10: Make xenconsoled ignore doms with qemu-dm

pv-qemu 10/10: Make xenconsoled ignore doms with qemu-dm

This patch writes a field /local/vm/DOMID/console/type taking the
value 'ioemu' or 'xenconsoled'. If xenconsoled sees a type that is
not its own, then it skips handling of that guest. The qemu-dm
process doesn't need to read this field since it will only attach
to the console if given the -serial pty arg which XenD already
ensures matches this xenstore field.

The overall behaviour is that if a paravirt guest has a qemu-dm
process running then that handles the console, otherwise the
xenconsoled handles it. The former is more functional, with the
exception of not currently supporting persistent logging to a
file at the same time as exposing a PTY.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>