KVM: selftests: Use kvm_cpu_has() for nested VMX checks

Use kvm_cpu_has() to check for nested VMX support, and drop the helpers
now that their functionality is trivial to implement.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-7-seanjc@google.com