hppa: Add emulation of Artist graphics

hppa: Add emulation of Artist graphics

This adds emulation of Artist graphics good enough to get a text
console on both Linux and HP-UX. The X11 server from HP-UX also works.

Adjust boot-serial-test to disable graphics, so that SeaBIOS outputs
to the serial port, as expected by the test.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Message-Id: <20191220211512.3289-6-svens@stackframe.org>
[rth: Merge Helge's test for machine->enable_graphics]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>