Allow customisation of the disk index separator

As per issue 124, some Xen versions (or packaging) don't deal nicely
with the colon being part of a disk name. Therefore we add a
configure-time option for customising this.

Note: setting the separator to interesting values like / is not
handled by the code. This being a configure-time option (e.g. to be
set by distribution packagers), we assume the person building the code
knows what they are doing.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>