Clean up seed image file

Since libvirt doesn't manage the seed's storage volume, it doesn't
currently get deleted when cleanup-env is run. This change handles the
destroy and volume cleanup differently for the seed such that the volume
gets removed.

Changes have also been added in the destroy and remove process to avoid
producing noisy error messages, by only destroying domains that are
currently running, as well as only attempting to remove storage for
non-seed domains.

Change-Id: I65b8f5dde936efa11c0b6857a5b8b138da7f425b
Closes-Bug: 1298466
Merge "Clean up seed image file"