ioemu: various fixes to `Use main memory for video memory'

ioemu: various fixes to `Use main memory for video memory'

- fix ioemu segv with old firmware
  Without notifying ioemu of address, ioemu will segv.

- fix qemu-dm segv with malicous firmware
  If notifying ioemu more than once, ioemu will segv.

Usually such cases don't happen, but malicious guest can
do it intentionally.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
(Cross-ported from xen-unstable
 18449:33d907ff2b043c4bff5c265737dab0bb52d6f773
this is a fix to the patch
 0844825b76924eac7719875b3886072b74e19397
which itself was cross-ported from xen-unstable
 18383:dade7f0bdc8d6b36b1914598d83c616ee5ce97cb

There were no conflicts or problems with patch -l.)