qvm-trim-template manually assemble device dict, fix it to include
also 'desc' field, as it is now checked.
Fixes ba4dec5 "qvm-block: fix checking attached status of disk image files"
Trim template name to fit full VM name in 31 chars.
At the same time, check if the VM already exists - if so - remove it
first (or error out asking the user to remove it manually - if VM isn't
marked as internal).
Now that VM is created as internal, to skip appmenus creation.
QubesOS/qubes-issues#1910FixesQubesOS/qubes-issues#1655
