Unable to start a KVM inside a KVM instance running with 4.15 KVM kernel

The kvm smoke test failed on a KVM system running 4.15.0-1004-kvm kernel.

Steps:
 1. Deploy a KVM instance with Bionic, install linux-kvm on it
 2. Run the following command for the ubuntu_kvm_smoke_test:
  git clone --depth=1 git://kernel.ubuntu.com/ubuntu/autotest-client-tests -b master-next
  git clone --depth=1 git://kernel.ubuntu.com/ubuntu/autotest
  rm -fr autotest/client/tests
  ln -sf ~/autotest-client-tests autotest/client/tests
  AUTOTEST_PATH=/home/ubuntu/autotest sudo -E autotest/client/autotest-local --verbose autotest/client/tests/ubuntu_kvm_smoke_test/control

Command:
  uvt-kvm create bjf-test release=xenial arch=amd64

Result:
  uvt-kvm: error: libvirt: Requested operation is not valid: network 'default' is not active

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-1004-kvm 4.15.0-1004.4
ProcVersionSignature: User Name 4.15.0-1004.4-kvm 4.15.15
Uname: Linux 4.15.0-1004-kvm x86_64
ApportVersion: 2.20.9-0ubuntu5
Architecture: amd64
Date: Wed Apr 18 10:34:51 2018
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: linux-kvm
UpgradeStatus: No upgrade log present (probably fresh install)