Add hamfisted locking to the Xen/PV pmap code: Only allow one thread to be in {pmap_pinit, pmap_copy, pmap_release} at a time.
Add hamfisted locking to the Xen/PV pmap code: Only allow one thread to
be in {pmap_pinit, pmap_copy, pmap_release} at a time.

This reduces the rate of panics when running 'make index' from ~0.6/hour
to ~0.02/hour (p < 10^-30).

At a later date this locking will be removed, and for this reason, it is
wrapped in #ifdef HAMFISTED_LOCKING; this temporary hack is being put in
place with the intention of shipping somewhat-stable Xen bits in FreeBSD
8.2-RELEASE.

PR:		kern/153672
MFC after:	3 days

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