kvm: allow tpr patching to write to write-protected vapic option rom

kvm: allow tpr patching to write to write-protected vapic option rom

Now that we allow the bios to write protect option roms, we need to allow
the tpr patching code to write to this write protected memory.  This means
using cpu_physical_memory_write_rom() instead of the usual APIs.

Fixes Windows XP without flexpriority.

Signed-off-by: Avi Kivity <avi@redhat.com>