MSR_KGSBASE is no longer saved and restored from the guest MSR save area. This
behavior was changed in r271888 so update the comment block to reflect this.

MSR_KGSBASE is accessible from the guest without triggering a VM-exit. The
permission bitmap for MSR_KGSBASE is modified by vmx_msr_guest_init() so get
rid of redundant code in vmx_vminit().
Add some more KTR events to help debugging.