ubuntu_nbd_smoke_test failed with KVM kernels

[Po-Hsu Lin]

This test failed with Bionic 4.15 KVM kernel:
  Cannot load Network Block Device module nbd.o
  stderr:
  modprobe: FATAL: Module nbd not found in directory /lib/modules/4.15.0-1021-kvm

When look into the config, in 4.15 KVM kernel it is (also applies to the 4.4 KVM kernel):
    # CONFIG_BLK_DEV_NBD is not set

But it's =m in Bionic kernel:
    # CONFIG_BLK_DEV_NBD is not set

So we will need to decide if we want to add this into the KVM kernels, or disable the test for KVM kernels

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-1021-kvm 4.15.0-1021.21
ProcVersionSignature: User Name 4.15.0-1021.21-kvm 4.15.18
Uname: Linux 4.15.0-1021-kvm x86_64
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
Date: Thu Sep 20 09:41:04 2018
SourcePackage: linux-kvm
UpgradeStatus: No upgrade log present (probably fresh install)