virsh: Don't open code virshEnumComplete()

virsh: Don't open code virshEnumComplete()

Now that we have a function that generates string list for given
enum, let's use that instead of open coding it.

Note, after this there are still some 'candidates' left (e.g,
virshNetworkEventNameCompleter(), or
virshNetworkUpdateCommandCompleter()). These are not converted
because either they don't have a convenient int2str function or
they don't start from the very beginning of the enum.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>