KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR

KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR

Return KVM_MSR_RET_UNSUPPORTED instead of '1' (which for all intents and
purposes means "invalid") when rejecting accesses to KVM PV MSRs to adhere
to KVM's ABI of allowing host reads and writes of '0' to MSRs that are
advertised to userspace via KVM_GET_MSR_INDEX_LIST, even if the vCPU model
doesn't support the MSR.

E.g. running a QEMU VM with

  -cpu host,-kvmclock,kvm-pv-enforce-cpuid

yields:

  qemu: error: failed to set MSR 0x12 to 0x0
  qemu: target/i386/kvm/kvm.c:3301: kvm_buf_set_msrs:
        Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Cc: stable@vger.kernel.org
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20251230205948.4094097-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>