r334533 renders head unbootable on XenServer 7.4

After trial and error, I found r334533 to be responsible for rendering 
head unbootable on XenServer 7.4. Neither of the two available 
XenServer patches has been applied on the host server.

Normal and verbose boot messages stops non-deterministically at:

hpet0: <High Precision Event Timer> iomem 0xfed00000-0xfed003ff on acpi0

Or at:

granttable0: <Xen Grant-table Device> on xenpv0

This is on a non-critical VM, but I think this issue should be 
resolved by the time stable/12 becomes available, or if stable/11 
will be impacted by a MFH of r334533 in the near future.

The host server will be upgraded to XenServer 7.5 by the end of the 
week, in case that makes the issue go away.

-- 
Trond.