XenAPI improve post snapshot coalesce detection

The coalesce detection is not working well after this change:
ae2a27ce19f3e24d4a8c713a73e617f4cd71d4b4

The snapshot operation will introduce a new VHD file into the VDI chain,
and in many cases, that would be coalesced during the next SR scan. So
we need to walk the chain after the snapshot has been taken, not before.

This fixes the cause of the mentioned bug, but it doesn't help with
launching the partially corrupted snapshots created because of this bug,
so this only partially fixes the bug.

Change-Id: I0bf7535ae3f7d9e6820f8dc07075892953d80a78
Partial-Bug: #1362595
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"
Merge "XenAPI improve post snapshot coalesce detection"