After the migration to libosinfo, choosing an OS variant from "New VM"
wizard became a mess for the very long list of options.
Using an autocomplete entry text will facilitate this task, as an user
can start typing the variant as soon as the OS type is selected.
The combo-box is left untouched, so undecided users can still use it.
UI-suggested-by: Máirín Duffy <duffy@redhat.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>