avahi: Don't complain with g_warning if the daemon wasn't running

avahi: Don't complain with g_warning if the daemon wasn't running

This is a normal case when running unit tests in client code
on continuous integration infrastructure. When those tests are
running they will set G_DEBUG=fatal-warnings which will cause
the program to abort if a warning is emitted. Instead, emit
a debug message if the problem was that we couldn't connect to
the daemon.

Closes: #1542
Approved by: jlebon