KVM: selftests: Verify get/set PERF_CAPABILITIES w/o guest PDMC behavior

Add another testcase to x86's PMU capabilities test to verify that KVM's
handling of userspace accesses to PERF_CAPABILITIES when the vCPU doesn't
support the MSR (per the vCPU's CPUID).  KVM's (newly established) ABI is
that userspace MSR accesses are subject to architectural existence checks,
but that if the MSR is advertised as supported _by KVM_, "bad" reads get
'0' and writes of '0' are always allowed.

Link: https://lore.kernel.org/r/20240802185511.305849-9-seanjc@google.com


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