libxl: remove "reg" and "enable" fields from PCI device.

libxl: remove "reg" and "enable" fields from PCI device.

The structure of the BDF argument used with PCI passthrough related
hypercalls was taken from the structure of the PCI config_address
register (I/O port 0xCF8) which allows I/O mapped access to PCI
configuration space but these fields have no meaning in the context of
PCI passthrough configuration and hence do not need to be exposed via
libxl.

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