Make use of the memory API's ability to satisfy multi-byte accesses via
multiple single-byte accesses.

We have to keep vga_mem_{read,write}b() since they're used by cirrus.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>