KVM: x86: Ignore benign host writes to "unsupported" F15H_PERF_CTL MSRs

Ignore host userspace writes of '0' to F15H_PERF_CTL MSRs KVM reports
in the MSR-to-save list, but the MSRs are ultimately unsupported.  All
MSRs in said list must be writable by userspace, e.g. if userspace sends
the list back at KVM without filtering out the MSRs it doesn't need.

Note, reads of said MSRs already have the desired behavior.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220611005755.753273-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>