Fix hvm guest cirrus_vga hvm s3 resume failure

Fix hvm guest cirrus_vga hvm s3 resume failure

This patch is for fixing hvm guest cirrus_vga hvm s3 resume failure.
 We found latest Qemu (after Mar 3.10) will cause HVM guest vga fails
to resume back from S3.  It's because vga_bios_init is not only needed
during init, but also needed after cirrus_vga reset (hvm s3 will do
qemu_reset).

Also, we found the outdated s->vram_ptr reset will cause s3 resume
screen black, so we remove it. (win2000 boot up does not need it now)

We verified the fix with the latest cs/qemu.

[ Patch modified slightly by Ian Jackson to reduce code motion. ]

Signed-off-by: Liping Ke<liping.ke@intel.com>
Signed-off-by: Ke Yu<ke.yu@intel.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>