Fail quickly if file injection for boot volume

Previously if the user attempted file injection on a booted volume it
would fail and retry several times, which slowed down the instance
spawning.  Instead of attempting file injection, fail quickly, and
log a warning

Backport to stable/grizzy

Fix bug 1188543

Conflicts:
	nova/virt/libvirt/driver.py

Change-Id: I264540de736c2bcb92567826fe5ba672e1244ba2
(cherry picked from commit f59c2d64ac8f0523c587e23b2d1c59fdfe7ea131)
Merge "Fail quickly if file injection for boot volume" into stable/grizzly