KVM: VMX: Don't allow uninhibited access to EFER on i386

(cherry picked from commit 16175a796d061833aacfbd9672235f2d2725df65)

vmx_set_msr() does not allow i386 guests to touch EFER, but they can still
do so through the default: label in the switch.  If they set EFER_LME, they
can oops the host.

Fix by having EFER access through the normal channel (which will check for
EFER_LME) even on i386.

Reported-and-tested-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>