KVM: PPC: Book3S HV: Add missing barriers to XIVE code and document them

KVM: PPC: Book3S HV: Add missing barriers to XIVE code and document them

This adds missing memory barriers to order updates/tests of
the virtual CPPR and MFRR, thus fixing a lost IPI problem.

While at it also document all barriers in this file.

This fixes a bug causing guest IPIs to occasionally get lost.  The
symptom then is hangs or stalls in the guest.

Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Tested-by: Guilherme G. Piccoli <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>