x86: make map_domain_page_global() a simple wrapper around vmap()

x86: make map_domain_page_global() a simple wrapper around vmap()

This is in order to reduce the number of fundamental mapping mechanisms
as well as to reduce the amount of code to be maintained. In the course
of this the virtual space available to vmap() is being grown from 16Gb
to 64Gb.

Note that this requires callers of unmap_domain_page_global() to no
longer pass misaligned pointers - map_domain_page_global() returns page
size aligned pointers, so unmappinmg should be done accordingly.
unmap_vcpu_info() violated this and is being adjusted here.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>