discover/udev: Register udev monitor before enumerating devices

discover/udev: Register udev monitor before enumerating devices

Currently, we enumerate udev devices before setting up our monitor. This
means that we may lose devices that udev discovers after we start the
enumeration, but before the monitor is registered.

This change enables the monitor before enumeration, so we don't lose
devices. We add a filter to the enumeration code to only parse
completely initialised devices.

This means we may need to handle change events as the main source of
device notifications. We keep the existing CDROM event handler, but
check for new devices and handle those as an add.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>