makes sure correct PCI device allocation

With this patch, on the compute node, a stats pool will be associated with a
list of devices that belongs to the pool. This makes sure that PCI devices are
allocated out of the same stats pools that are used by the nova scheduler to
satisfy the PCI requests. And therefore, stats pools on the compute nodes will
be kept in sync with their counterparts in the nova scheduler.

Change-Id: I2d97c6354215e2ac5ff659e3203c33771abe1c09
Closes-bug: #1288809
Merge "makes sure correct PCI device allocation"