Doesn't support 2560x1600, which can be used when virtualized by qemu

[MattColyer]

So I virtualize Ubuntu 9.10 inside of qemu-kvm on a 2560x1600 monitor. It turns out that while the qemu VESA driver can support this resolution (and reports it) the X server disregards this mode as invalid because there is a hard limit of 2048 in the VESA X driver.

I have included a patch which changes this limit. I realize that this problem isn't really Ubuntu specific but I was hoping the change might go upstream from here.