CA-86869: Don't list snapshots when listing custom templates.

print-custom-templates is used by xe-backup-metadata so that it can then
backup metadata of all custom templates. xapi explicitly blocks exporting
the metadata of snapshots, so there's no reason for
print-custom-templates to include snapshots.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1815 from euanh/CA86869

CA-86869: Don't list snapshots when listing custom templates.