Fix crash on resolution change -> screen dump -> vga redraw (Avi Kivity)

The vga screen dump function updates last_width and last_height,
but does not change the DisplaySurface that these variables describe.
A consequent vga_draw_graphic() will therefore fail to resize the
surface and crash.

Fix by invalidating the display state after a screen dump, forcing
vga_draw_graphic() to reallocate the DisplaySurface.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7026 c046a42c-6fe2-441c-8c8c-71466251a162
