[networking-guide] Update details on pci_alias
[networking-guide] Update details on pci_alias

Adding the missing details to generate pci request using pci_alias
in the SR-IOV guide.

Change-Id: I8243419c91118c4af1992b85cefb62a4f9197743
Closes-Bug: #1583859

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals