Fix remaining missing changes to update to xen-unstable 17958.

Fix remaining missing changes to update to xen-unstable 17958.

We had accidentally dropped some changes from 17737 to 17831.
Fixed by in xen-unstable:
  hg update 17737
  patch -p1 < vga-patch-from-xen-unstable-17737:c93a913c221f
  hg diff -r 17831 tools/ioemu >patch
and then in qemu.git:
  patch -R -U -p3 <.../patch
and fix up conflicts, which were:
  Makefile.target - stubdom changes, defer for now
  hw/vga.c - one easy fixup, one change already in our tree
  hw/xenfb.c - already done completely earlier, so discard changes