x86/ucode: Fix errors with start/end_update()

x86/ucode: Fix errors with start/end_update()

c/s 9267a439c "x86/ucode: Document the behaviour of the microcode_ops hooks"
identified several poor behaviours of the start_update()/end_update_percpu()
hooks.

AMD have subsequently confirmed that OSVW don't, and are not expected to,
change across a microcode load, rendering all of this complexity unecessary.

Instead of fixing up the logic to not leave the OSVW state reset in a number
of corner cases, delete the logic entirely.

This in turn allows for the removal of the poorly-named 'start_update'
parameter to microcode_update_one(), and for svm_host_osvw_{init,reset}() to
become static.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Paul Durrant <paul@xen.org>