Increase cpu time for image conversion

Apparently the current 8 second timeout on qemu-info may not be
sufficient if snapshot images are > 120G in size.

This bumps that to 30s instead to provide a backstop, but not hurt
people with large snapshots.

Change-Id: I877b9401a671904a13bb07bae3636b72d7d20df8
Closes-Bug: #1705340

nova/virt/images.py

@@ -39,7 +39,7 @@
 IMAGE_API = image.API()
 
 QEMU_IMG_LIMITS = processutils.ProcessLimits(
-    cpu_time=8,
+    cpu_time=30,
     address_space=1 * units.Gi)