vl: Assert drive_new() does not fail in default_drive()

If creating (empty) default drives fails, it's a bug.  Therefore,
assert() is more appropriate than exit(1).

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20181017082702.5581-33-armbru@redhat.com>

vl.c

@@ -1156,9 +1156,7 @@ static void default_drive(int enable, int snapshot, BlockInterfaceType type,
     }
 
     dinfo = drive_new(opts, type);
-    if (!dinfo) {
-        exit(1);
-    }
+    assert(dinfo);
     dinfo->is_default = true;
 
 }