vga: Add support for VGA text buffer output on x86

vga: Add support for VGA text buffer output on x86

Hypervisor messages are useful for debugging and are
typically handed out to the serial port. Unfortunately, x86
computers often lack of a serial port. This patch allows
hypervisor messages to be redirected to a screen by leveraging
the traditional VGA text buffer mode.

Signed-off-by: Daniel Sangorrin <daniel.sangorrin@toshiba.co.jp>
[Jan: avoid row_line writeback in panic case, remove redundant braces]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>