console: buffer and show origin of guest PV writes

console: buffer and show origin of guest PV writes

Guests other than domain 0 using the console output have previously been
controlled by the VERBOSE #define, but with no designation of which
guest's output was on the console. This patch converts the HVM output
buffering to be used by all domains except the hardware domain (dom0):
stripping non-printable characters, line buffering the output, and
prefixing it with the domain ID. This is especially useful for debugging
stub domains during early boot.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Keir Fraser <keir@xen.org>