KVM: nVMX: Validate the EPTP when emulating INVEPT(EXTENT_CONTEXT)

Signal VM-Fail for the single-context variant of INVEPT if the specified
EPTP is invalid.  Per the INEVPT pseudocode in Intel's SDM, it's subject
to the standard EPT checks:

  If VM entry with the "enable EPT" VM execution control set to 1 would
  fail due to the EPTP value then VMfail(Invalid operand to INVEPT/INVVPID);

Fixes: bfd0a56b ("nEPT: Nested INVEPT")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-3-sean.j.christopherson@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>