KVM: selftests: Check that PV_UNHALT is cleared when HLT exiting is disabled

KVM_FEATURE_PV_UNHALT is expected to get cleared from KVM PV feature CPUID
data when KVM_X86_DISABLE_EXITS_HLT is enabled. Add the corresponding test
to kvm_pv_test.

Note, the newly added code doesn't actually test KVM_FEATURE_PV_UNHALT and
KVM_X86_DISABLE_EXITS_HLT features.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240228101837.93642-4-vkuznets@redhat.com
[sean: add and use vcpu_cpuid_has()]
Signed-off-by: Sean Christopherson <seanjc@google.com>