xen/setup: Work properly with 'dom0_mem=X' or with not dom0_mem.

xen/setup: Work properly with 'dom0_mem=X' or with not dom0_mem.

We ignored the X value and ended up populating up to
max(MAX_DOMAIN_PAGES, last E820_RAM entry).

This fixes it by figuring out how many RAM nr_pages the
hypervisor wanted to provide to us and cap the populate
hypercalls up to that.

The end result is (on a 8GB box):

dom0_mem=1G
-Memory: 610884k/9435136k available (5817k kernel code, 1136060k absent, 7688192k reserved, 2899k data, 696k init)
+Memory: 724184k/1053064k available (5817k kernel code, 4552k absent, 324328k reserved, 2899k data, 696k init)

no dom0_mem
-Memory: 7619036k/9435136k available (5817k kernel code, 1136060k absent, 680040k reserved, 2899k data, 696k init)
+Memory: 7621460k/9208688k available (5817k kernel code, 1136060k absent, 451168k reserved, 2899k data, 696k init)

[v1: Details added]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>