vbe: rework sanity checks

Plug a bunch of holes in the bochs dispi interface parameter checking.
Add a function doing verification on all registers.  Call that
unconditionally on every register write.  That way we should catch
everything, even changing one register affecting the valid range of
another register.

Some of the holes have been added by commit
e9c6149f6ae6873f14a12eea554925b6aa4c4dec.  Before that commit the
maximum possible framebuffer (VBE_DISPI_MAX_XRES * VBE_DISPI_MAX_YRES *
32 bpp) has been smaller than the qemu vga memory (8MB) and the checking
for VBE_DISPI_MAX_XRES + VBE_DISPI_MAX_YRES + VBE_DISPI_MAX_BPP was ok.

Some of the holes have been there forever, such as
VBE_DISPI_INDEX_X_OFFSET and VBE_DISPI_INDEX_Y_OFFSET register writes
lacking any verification.

Security impact:

(1) Guest can make the ui (gtk/vnc/...) use memory rages outside the vga
frame buffer as source  ->  host memory leak.  Memory isn't leaked to
the guest but to the vnc client though.

(2) Qemu will segfault in case the memory range happens to include
unmapped areas  ->  Guest can DoS itself.

The guest can not modify host memory, so I don't think this can be used
by the guest to escape.

CVE-2014-3615

Cc: qemu-stable@nongnu.org
Cc: secalert@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
