Do not unconditionally call clear_atomic_switch_msr() when updating
EFER. This adds up to four unnecessary VMWrites in the case where
guest_efer != host_efer, e.g. if the load_on_{entry,exit} bits were
already set.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Reviewed-by: Jim Mattson <jmattson@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>