virinitctl: Expose fifo paths and allow caller to chose one

virinitctl: Expose fifo paths and allow caller to chose one

So far the virInitctlSetRunLevel() is fully automatic. It finds
the correct fifo to use to talk to the init and it will set the
desired runlevel. Well, callers (so far there is just one) will
need to inspect the fifo a bit just before the runlevel is set.
Therefore, expose the internal list of fifos and also allow
caller to explicitly use one.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>