xen/mmu: Move the setting of pvops.write_cr3 to later phase in bootup.

We move the setting of write_cr3 from the early bootup variant
(see git commit 0cc9129d


"x86-64, xen, mmu: Provide an early version of write_cr3.")
to a more appropiate location.

This new location sets all of the other non-early variants
of pvops calls - and most importantly is before the
alternative_asm mechanism kicks in.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>