hvm stdvga: Do not emulate PIO reads inside Xen. They should be rare

hvm stdvga: Do not emulate PIO reads inside Xen. They should be rare
(PIO write emulation is really for book-keeping to detect when we
enter/leave stdvga mode, and to work out what to do with mmio
accesses), and we may do the wrong thing depending on emulated SVGA
hardware and current mode.

This simplifies the code and means that 'stdvga=1' once again works
properly and causes the stdvga bios to be loaded by hvmloader.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>