formats: Alwas use virgl_format, because pipe_format is mesa internal, and just changed

Mesa moved away from the manual assignment of numbers to the pipe_format enums, so that the numbers used in virglrenderer are now out of sync and we can no longer rely on a stable numbering (for details see https://bugs.freedesktop.org/show_bug.cgi?id=111454). This is probably the right moment to settle on virgl_format for all use cases. The translation from pipe_format to virgl_format has to be done in the guest anyway, because we have to be compatible with new and old guests.