Add ability to select or exclude VMs und CTs via label

[zrav]

Currently we can only select or exclude VMs and CTs from snapshotting via IDs. It would be great if we could do this via labels too. For instance, I could want to snapshot all VMs and CTs except those labeled "nosnap". So I could image an argument called --exclude-label. Or a complementary --include-label, which could be used instead of or in addition to --vmid.