Add a go.mod file to make build work with go 1.16
Add a go.mod file to make build work with go 1.16

Go 1.16 now expects modules to be defined if not building in a
normal $GOPATH/src location. This results in errors when
attempting to build

  $ go build
  go: cannot find main module, but found .git/config in /builds/user/libvirt-go
       to create a module there, run:
       go mod init

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go,libvirt/libvirt-go