pv-qemu 8/10: Add pv console to QEMU paravirt machine

pv-qemu 8/10: Add pv console to QEMU paravirt machine

This patch adds a paravirt console driver to qemu-dm. This is used
when the QEMU machine type is 'xenpv', connecting to the ring buffer
provided by the guest kernel. The '-serial' command line flag controls
how the guest console is exposed.

For parity with xenconsoled the '-serial pty' arg can be used. For
guests which are running a qemu-dm device model, the xenconsoled
daemon is no longer needed for guest consoles. The code for the
xen_console.c is based on the original code in
tools/console/daemon/io.c, but simplified; since its only dealing with
a single guest there's no state tracking to worry about.

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