pseries: Consolidate hack for RTAS display-character usage

Currently the pseries machine contains not one but two somewhat ugly hacks
to allow printing of early debug messages before the guest has properly
read the device tree.

First, we special case H_PUT_TERM_CHAR so that a vtermno of 0 (usually
invalid) will look for a suitable vty and use that.  This supports Linux's
early debug code which will use H_PUT_TERM_CHAR with vtermno==0 before
reading the device tree.  Second, we support the RTAS display-character call.
This takes no vtermno so we assume the address of the default first VTY.

This patch makes things more consistent by folding the second hack into the
first.  Now, display-character uses the existing vty_lookup() function to
do the same search for a suitable VTY.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andreas Färber <afaerber@suse.de>
