Qemu: display: cirrus_vga: a divide by zero in cirrus_do_copy

Quick emulator Qemu built with the Cirrus CLGD 54xx VGA Emulator support is vulnerable to a divide by zero issue. It could occur while copying VGA data when cirrus graphics mode was set to be VGA. A privileged user inside guest could use this flaw to crash the Qemu process instance on the host,...