XenAPI: Add the mechanism to attach a pci device to a VM.

 Before booting a VM we check if there are PCI devices reserved
by this VM and if there are any we attach them.

 A PCI device can be asked by a VM by choosing a flavor that
request PCI devices. See the wiki about pci passthrough for more
details:

https://wiki.openstack.org/wiki/Pci_passthrough#Create_a_flavor

 We are attaching a PCI device by adding a key "pci" to the
"other-config" parameter of a VM.

Change-Id: I4aca96b11f931f70cf6442adac64e867ce1affb6
Implements: blueprint pci-passthrough-xenapi
Merge "XenAPI: Add the mechanism to attach a pci device to a VM."