The general documentation we have for pv_ops is currenty present
on the IA64 docs, but since this documentation covers IA64 xen
enablement and IA64 Xen support got ripped out a while ago
through commit d52eefb47 present since v3.14-rc1 lets just
simplify, generalize and move the pv_ops documentation to a
shared place.

Cc: Isaku Yamahata
Cc: Jeremy Fitzhardinge
Cc: Chris Wright
Cc: Alok Kataria
Cc: Rusty Russell
Cc: virtualization@lists.linux-foundation.org
Cc: Tony Luck
Cc: Fenghua Yu
Cc: Boris Ostrovsky
Cc: xen-devel@lists.xenproject.org
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Luis R. Rodriguez
Signed-off-by: Rusty Russell