Convert all vga memory to the memory API.  Note we need to fall back to
get_system_memory(), since the various buses don't pass the vga window
as a memory region.

We no longer need to sync the dirty bitmap of the cirrus mapped memory
banks, since the memory API takes care of that for us.

[jan: fix vga-pci logging]
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>