KVM: x86: clflushopt should be treated as a no-op by emulation

The instruction emulator ignores clflush instructions, yet fails to
support clflushopt. Treat both similarly.

Fixes: 13e457e0

 ("KVM: x86: Emulator does not decode clflush well")
Signed-off-by: David Edmondson <david.edmondson@oracle.com>
Message-Id: <20201103120400.240882-1-david.edmondson@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>