engine: change mVerbs to Set (2/N)

mVerbs does not have to be a hashset since only the key is checked and
the value is completely ignored, while the value is always the same as
the key.

Change-Id: I8d97f79e1aaf4818cc58918d19f1b0e9055d151e
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>