kvm: x86: Add Intel PMU MSRs to msrs_to_save[]

These MSRs should be enumerated by KVM_GET_MSR_INDEX_LIST, so that
userspace knows that these MSRs may be part of the vCPU state.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Eric Hankland <ehankland@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>