i386: kvm: Add support for MSR filtering

i386: kvm: Add support for MSR filtering

KVM has grown support to deflect arbitrary MSRs to user space since
Linux 5.10. For now we don't expect to make a lot of use of this
feature, so let's expose it the easiest way possible: With up to 16
individually maskable MSRs.

This patch adds a kvm_filter_msr() function that other code can call
to install a hook on KVM MSR reads or writes.

Signed-off-by: Alexander Graf <agraf@csgraf.de>
Message-Id: <20221004225643.65036-3-agraf@csgraf.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>