selftests: kvm: add _vm_ioctl

As in kvm_ioctl and _kvm_ioctl, add
the respective _vm_ioctl for vm_ioctl.

_vm_ioctl invokes an ioctl using the vm fd,
leaving the caller to test the result.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210318151624.490861-1-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>