Support both GPU PCI bus address formats

The NVIDIA procfs file currently uses the format:

xxxx:xx.xx.x

But everywhere else (lspci, sysfs, xapi/xenopsd) uses the format:

xxxx:xx:xx.x

This change means that the NVIDIA unbind code understands both formats,
in case the procfs file changes in future to use the format used
elsewhere.

Signed-off-by: John Else <john.else@citrix.com>