xl: Document VGA problems arising from lack of physmap dmop

xl: Document VGA problems arising from lack of physmap dmop

Ross reports that stdvga guests do not work, and cirrus guests are
slow, because qemu tries to do xc_domain_add_to_physmap.  We will need
another dmop to fix this properly.

For now, document the problem.

(In the cirrus case, the vram remains mapped at the old guest-physical
addresses, while the guest runs.  We are not sure whether this is a
correctness or security problem and we should advise against it.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reported-by: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Paul Durrant <Paul.Durrant@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>