Add use existing vm option to boot-seed-vm

In CI testing, the environment in libvirt will already exist, adding
the --use-existing-vm option to indicate this and so it doesn't run
the virsh commands.

Still defaults to being configured.

Change-Id: If5c156c833509b2774e3e6c775498a4f84d8442b
Merge "Add use existing vm option to boot-seed-vm"