x86/xen: move shared page setup to early init handler

As done with the hypercall page, move the setup fo the shared info page into
the newly introduced helper, which the aim of having a single helper and call
site used by both HVM and PV in order to setup the basic Xen environment.

Sponsored by: Cloud Software Group
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D43933