xl: disk parsing preparation for empty cdrom devices

xl: disk parsing preparation for empty cdrom devices

Prepare the ground for parsing the xend empty cdrom syntax, by
separating out some non-functional changes as this pre-patch:

* Clarify the disk syntax documentation wording to refer to deprecated
  syntaxes too.

* Make DPC in libxlu_disk_l.l useable in the helper functions as well
  as in lexer rules, by providing two definitions, each in force in
  the appropriate parts of the file.

* Break the <vdev>[:<devtype>] parsing out into a helper function,
  `vdev_and_devtype'.

No functional change.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>