KVM: nSVM: check for EFER.SVME=1 before entering guest

EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1 and hides
an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check manually
to detect invalid guest state.

Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>