hwdb: implement --root option for systemd-hwdb query
Currently, the systemd-hwdb --root flag only has an effect for the
'update' verb. It would be useful to be able to use the --root option
for the 'query' verb too (e.g. for testing a hwdb.bin created with
systemd-hwdb update --root <path>).
Use sd_hwdb_new_from_path to initialize the hwdb if --root is passed to
systemd-hwdb query.
Note that this functionality was not added to 'udevadm hwdb' since that
command is deprecated.