This is a simpler solution to 869981, where migration breaks since qxl's
rom bar size has changed. Instead of ignoring fields in QXLRom, which is what has
actually changed, we remove some of the modes, a mechanism already
accounted for by the guest. The modes left allow for portrait and
landscape only modes, corresponding to orientations 0 and 1.
Orientations 2 and 3 are dropped.

Added assert so that rom size will fit the future QXLRom increases via
spice-protocol changes.

This patch has been tested with 6.1.0.10015. With the newer 6.1.0.10016
there are problems with both "(flipped)" modes prior to the patch, and
the patch loses the ability to set "Portrait" modes. But this is a
separate bug to be fixed in the driver, and besides the patch doesn't
affect the new arbitrary mode setting functionality.
Signed-off-by: NAlon Levy <alevy@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>