SGABIOS: fix wrong video attrs for int 10h, ah==13h

Fix Bug #1670509: wrong video attrs for int 10h, ah==13h
The subroutine set_cursor_position is trashing %bx.
Unfortunately, %bl contains the video attribute for write_string.

The fix saves %bx in the function prolog and restores it in
the epilog.

Signed-off-by: Herbie Robinson <Herbie.Robinson@stratus.com>
Message-Id: <1489161295-23864-1-git-send-email-Herbie.Robinson@stratus.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>