KVM: x86: Add infrastructure for MSR filtering

In the following commits we will add pieces of MSR filtering.
To ensure that code compiles even with the feature half-merged, let's add
a few stubs and struct definitions before the real patches start.

Signed-off-by: Alexander Graf <graf@amazon.com>

Message-Id: <20200925143422.21718-4-graf@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>