libxl: add all pci devices to xenstore at once (during VM create)

libxl: add all pci devices to xenstore at once (during VM create)

When adding pci devices one by one, pciback notice only the first one. For
every next, "state" is left as is (usualy "4" in that time), so backend will
not rescan xenstore. So when VM is starting all devices should be added at once
and then backend can initialize it.

This applies only to pci, because only pci backend have one xenstore dir for
multiple devices.

Signed-off-by: Marek Marczykowski <marmarek@mimuw.edu.pl>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>