x86/ACPI: restore VESA mode upon resume from S3

In order for "acpi_sleep=s3_mode" to have any effect, we should record
the video mode we switched to during boot. Since right now there's mode
setting code for VESA modes only in the resume case, record the mode
just in that one case.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I'm wondering actually whether the user having to explicitly request the
mode restoration is a good model: Why would we _not_ want to restore the
mode we've set during boot? In the worst case Dom0 kernel or X will
change the mode another time.

--- a/xen/arch/x86/boot/video.S
+++ b/xen/arch/x86/boot/video.S
@@ -455,14 +455,17 @@ check_vesa:
         cmpb    $0x99, %al
         jnz     _setbad                 # Doh! No linear frame buffer.
 
+        pushw   %bx
         subb    $VIDEO_FIRST_VESA>>8, %bh
         orw     $0x4000, %bx            # Use linear frame buffer
         movw    $0x4f02, %ax            # VESA BIOS mode set call
         int     $0x10
+        popw    %bx
         cmpw    $0x004f, %ax            # AL=4f if implemented
         jnz     _setbad                 # AH=0 if OK
 
         movb    $1, bootsym(graphic_mode)  # flag graphic mode
+        movw    %bx, bootsym(video_mode)
         stc
         ret