libvirt: fully parse PCI vendor/product IDs to integer data type

When parsing node device XML, the PCI vendor/product IDs were left
in their string form, which is unhelpful for later code wishing to
use their raw values. Fully parse them to the integer form instead.

Blueprint: virt-driver-get-available-resources-object
Change-Id: I46b9c33e08122eb7c28601752ecfc652e09175c1
Merge "libvirt: fully parse PCI vendor/product IDs to integer data type"