XML-RPC: add a method to show tag details
XML-RPC: add a method to show tag details

Allows to list devices tagged with the given tag.

Change-Id: Ia2fe4d49d9a9436443b071e4fcd1ab8ccfb9cfd4

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server