KVM: x86: Move MPK feature detection to common code

Both Intel and AMD support (MPK) Memory Protection Key feature.
Move the feature detection from VMX to the common code. It should
work for both the platforms now.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Message-Id: <158932795627.44260.15144185478040178638.stgit@naples-babu.amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>