Add a shadow VRAM to track changes to the real VRAM. When the guest

Add a shadow VRAM to track changes to the real VRAM.  When the guest
OS was given write access to the VRAM the device model tracked all
VRAM changes by updating the entire screen on every output loop,
causing significant overhead (a CPU bound loop in a guest slows down
by about 35%) and significant mouse latency (VNC uses the same data
path for mouse events and video updates).  With the shadow VRAM only
modified pages need to be updated and the comparison of the shadow
VRAM to the real VRAM only adds ~4% overhead while eliminating the
mouse latencies.

Signed-off-by: Don Dugger <donald.d.dugger@intel.com>