add get_devices() function?

This is somewhat related to #30, but different in my eyes.
DEFAULT_DEVICE or get_default_device() (depending on outcome of #30) is about the actual default and should stay as a feature due to haveing discid_get_default_device() returning a single string in libdiscid.

get_devices() would get a list of currently (or in general) available devices, which probably should include the one default.
This function is basically implemented in https://github.com/musicbrainz/picard/blob/master/picard/util/cdrom.py (as a list of currently available devices) for Windows and Linux.
An implementation for Mac OS X is partly implemented in isrcsubmit:
https://github.com/JonnyJD/musicbrainz-isrcsubmit/blob/bdb8efc2a6819d791dce198879bde34846c95d07/isrcsubmit.py#L301 (using drutil)

Part of this is implemented in libdiscid for Mac (for default_device) and proposed for Windows:
metabrainz/libdiscid#20.

Start by reading the decision and scope in #30, then compare the Windows and Linux approach in picard/util/cdrom.py with the Mac OS X approach at isrcsubmit.py#L301. Review the related libdiscid work for Mac and Windows. Done means agreeing on supported platforms and exposing a get_devices() result that includes the default device where applicable.