support multiple pv consoles for hvm guests too

support multiple pv consoles for hvm guests too

This patch allows qemu to provide pv console backends for hvm guests too:

- initialize the pv console backends when emulating a "xen_fv" machine;

- separate the pv consoles and emulated serials on xenstore: pv
consoles use the console prefix and emulated serials use the serial
prefix;

- read the "output" node on xenstore to know where the output of a pv
console has to go.

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