KVM: x86: Reject userspace attempts to access ARCH_CAPABILITIES w/o support

Reject userspace accesses to ARCH_CAPABILITIES if the MSR isn't supposed
to exist, according to guest CPUID.  However, "reject" accesses with
KVM_MSR_RET_UNSUPPORTED, so that reads get '0' and writes of '0' are
ignored if KVM advertised support ARCH_CAPABILITIES.

KVM's ABI is that userspace must set guest CPUID prior to setting MSRs,
and that setting MSRs that aren't supposed exist is disallowed (modulo the
'0' exemption).

Link: https://lore.kernel.org/r/20240802185511.305849-7-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>