x86, hvm: stdvga cache always on

x86, hvm: stdvga cache always on

currently the hypervisor vga cache (stdvga.c) enables itself only in
graphical mode and in the a0000h-affffh range. However there is no
reason for this: it already allocates enought memory to map the whole
vram. I am attaching a patch that implements the bank switching
mechanism in stdvga.c, allowing the cache to be always enabled when
the emulated graphic card is in VGA mode.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>