python/xc: reinstate original implementation of next_bdf

python/xc: reinstate original implementation of next_bdf

I missed the fact that next_bdf is used to parsed user supplied
strings when reviewing. The user supplied string is a NULL-terminated
string separated by comma. User can supply several PCI devices in that
string. There is, however, no delimiter for different devices, hence
we can't change the syntax of that string.

This patch reinstate the original implementation of next_bdf to
preserve the original syntax. The last argument for xc_assign_device
is always 0.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>