gvolumemonitor: Use GIOModule methods for getting default type
gvolumemonitor: Use GIOModule methods for getting default type

This fixes use of `GIO_USE_VOLUME_MONITOR=help`, and simplifies the
code. The reason this wasn’t used already seems to just be because it
was missed when `_g_io_module_get_default_type()` was introduced in
2013. The previous `get_default_native_class()` code in
`gunionvolumemonitor.c` was introduced in 2007.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1881

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib