hvm: Fix intra-vga-mem mmio in stdvga.c

hvm: Fix intra-vga-mem mmio in stdvga.c

The current stdvga code never checks if the copy_from/to_guest calls
fail, and thus silenty fails if both sides of an IOREQ_COPY points
into mmio space. This patch fixes this and makes the code correctly
handle intra-vga-mem copies, as well as log & turn off caching if some
more "exotic" type of mmio (say, copy from vga mem to some other
device's iomem), so that such mmio reqs will fall through to regular
ioemu where they can hopefully be handled correctly.

In practice, this patch fixes a number of graphical glitches for
guests running in standard vga mode, including a corrupted OS/2 boot
graphic.

Signed-off-by: Trolle Selander <trolle.selander@eu.citrix.com>
Signed-off-by: Keir Fraser <keir.fraser@eu.citrix.com>