x86/PV: fix unintended dependency of m2p-strict mode on migration-v2

x86/PV: fix unintended dependency of m2p-strict mode on migration-v2

This went unnoticed until a backport of this to an older Xen got used,
causing migration of guests enabling this VM assist to fail, because
page table pinning there precedes vCPU context loading, and hence L4
tables get initialized for the wrong mode. Fix this by post-processing
L4 tables when setting the intended VM assist flags for the guest.

Note that this leaves in place a dependency on vCPU 0 getting its guest
context restored first, but afaict the logic here is not the only thing
depending on that.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>