KVM: selftests: Convert vmx_close_while_nested_test away from VCPU_ID

Convert vmx_close_while_nested_test to use vm_create_with_one_vcpu() and
pass around a 'struct kvm_vcpu' object instead of using a global VCPU_ID.
Note, this is a "functional" change in the sense that the test now
creates a vCPU with vcpu_id==0 instead of vcpu_id==5.  The non-zero
VCPU_ID was 100% arbitrary and added little to no validation coverage.
If testing non-zero vCPU IDs is desirable for generic tests, that can be
done in the future by tweaking the VM creation helpers.

Opportunistically make the "vm" variable local, it is unused outside of
main().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>