Cause: qemu-kvm didn't have spice/qxl high resolution modes enabled.
Consequence: qemu-kvm did not allow a user to select a resolution higher than 1920x1080, which may have been rather limiting.
Fix: Remove the QXL_HIRES_MODES #define. Make the highres modes depend on the size of the vga ram (and draw area) instead. This enables some of the highres modes as RHEL-6.0 has VGA_RAM_SIZE set to 16 MB (unlike upstream qemu which has 8MB only).
Result: maximum supported resolution of spice/qxl increased to 2560x1600.