[Lucid ec2] Fix kernel deadlock in spinlock

[Stefan Bader]

Xen allows for a certain level of nested spinlocks. However the code
(SUSE patchset) that currently is in ec2 has a serious problems there.

I picked the new implementation from the more recent version of the
patchset and backported it to lucid-ec2. Since the bug is not easy
to reproduce it took a while to get feedback. But it seems to be
good enough according to a recent feedback.

NOTE: This requires an ABI bump!

-Stefan