Instead of having (current)three command line switches -std-vga,
-cirrusvga and -vmwarevga, provide one -vga switch which takes
an argument, so that:
qemu -std-vga   becomes qemu -vga std
qemu -cirrusvga becomes qemu -vga cirrus
qemu -vmwarevga becomes qemu -vga vmware

Update documentation accordingly.

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5335 c046a42c-6fe2-441c-8c8c-71466251a162