KVM: selftests: Use vcpu_{set,clear}_cpuid_feature() in nVMX state test

Use vcpu_{set,clear}_cpuid_feature() to toggle nested VMX support in the
vCPU CPUID module in the nVMX state test.  Drop CPUID_VMX as there are
no longer any users.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-29-seanjc@google.com