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 <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: herbierobinson

sgabios.S

@@ -1728,6 +1728,7 @@ write_string_tail:
 
 set_cursor_position:
   pushw %ax
+  pushw %bx
   pushw %ds
   pushw $BDA_SEG
   popw %ds                             /* ds = 0x40 */
@@ -1738,6 +1739,7 @@ set_cursor_position:
   movw %ax, %bx                        /* bx = cursor save offset */
   movw %dx, (%bx)                      /* save new cursor value */
   popw %ds
+  popw %bx
   popw %ax
   ret