xen: support attaching to additional consoles

The initial console is special -- it is set up by the domain builder
so 'printf' can be used for early-ish debugging. Additional consoles
can be hotplugged into a guest as regular devices.

We support the following Console.connect naming convention:

"0" | "" -> initial console
"n" when int_of_string(n) > 0 -> the device/console/n device
"x" -> the device/console device which has "name=<x>" in xenstore

Signed-off-by: David Scott <dave.scott@citrix.com>