libxl: disks: new xlu_disk_parse function

libxl: disks: new xlu_disk_parse function

Introduce new flex/regexp-based parser for disk configuration strings.
Callers will be updated in following patches.

The existing xm command line syntax for block-attach expects multiple
arguments containing different parameters for different parts of the
disk specification, so we supply a parser function which can take
multiple strings and scan them in sequence.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>