xenapi: fix bfv behavior when SR is not attached
xenapi: fix bfv behavior when SR is not attached

Fixes bug 1063920
If we try to boot from volume right after a detach volume operation
on the xensm volume driver, the SR has been disconnected. This results
in a failure to populate the vdis list. Introducing the SR if it
isn't in the xapi db fixes this.

Change-Id: I7e6b5e643ea3e20ba691141950cddfcb0ab5f3b8

n0ano/ganttclient