x86: test VMCS in memory after VMCLEAR

The Intel SDM states that following a VMCLEAR instruction, the VMCS
given as an operand to VMCLEAR should be resident in memory. If the
CPU caches VMCS data outside of memory, VMCLEAR must flush this cache
to memory.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20170421005004.137260-12-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>