hw/display/qxl: fix signed to unsigned comparison

Several small signedness / overflow corrections to qxl_create_guest_primary:
1. use 64 bit unsigned for size to avoid overflow possible from two 32
bit multiplicants.
2. correct sign for requested_height
3. add a more verbose error message when setting guest bug state (which
causes a complete guess blackout until reset, so it helps if it is
verbose).

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
