Emit remove events when loading contexts

[hughsie]

If a program, say fwupd, has references to existing devices it is probably not expected for the physical backing device to just vanish.

[hughsie]

@fdanis-oss i've respun this to only remove devices matching the tag (or the behavior as before if the tag is NULL) -- i think this is what we want if we need to support devices that replug with a different VID/PID.

[fdanis-oss]

Seems OK to me