pci: add API to get a BAR's mapped address

Some (hacky) devices that have a back-channel to read this
address back outside the normal configuration mechanisms, such
as VMware svga.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
