Mitigating performance impact with getting pci requests from DB

This change mitigates the performance impact in host manager that
retrieves an instance's PCI requests from DB while consuming it. This
is achieved by adding pci_requests into the instance dict before
calling consume_from_instance() and deleting it afterwards. The
complete solution will be provided when bug 1368260 is fully
addressed. The patch also removes the unnecessary context argument.

Change-Id: Ib0d12cde1c297a04e5d626b28d0a994c5dd4d965
Partial-Bug: 1368260
Merge "Mitigating performance impact with getting pci requests from DB"