libxl: Check whether a PCI device is assignable before assigning it do a domU

libxl: Check whether a PCI device is assignable before assigning it do a domU

Implement a new libxl function libxl_device_pci_list_assignable. This is
used to implement the xl list-assignable-pci-devices command and part of
the implementation is used to make sure that PCI devices are not multiply
assigned to one or more domU's before doing the passthrough assignment.

The function libxl_device_pci_list changes to libxl_device_pci_list_assigned
due to a parameter change for consistency with pci_list_assignable.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>