Clone properties from DispVM template after setting base properties
(qid, name, uuid). This means we can use standard clone_properties()
function. Otherwise various setters may fail - for example
netvm setter require uuid property initialized (for VM lookup in VM
collection).
Also, make dispvm_allowed check more robust - include direct creation of
DispVM, and also check just before VM startup (if property was changed
in the meantime).
FixesQubesOS/qubes-issues#3057
c247ddff72