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 32GB box):

-Memory: 31779964k/34603008k available (5831k kernel code, 1351620k absent, 1471424k reserved, 2886k data, 692k init)
-(XEN) memory.c:133:d0 Could not allocate order=0 extent: id=0 memflags=0 (15 of 512)
+Memory: 31788256k/34032852k available (5831k kernel code, 1351620k absent, 892976k reserved, 2886k data, 692k init)

with dom0_mem=1G
-Memory: 550608k/12890412k available (5831k kernel code, 1351620k absent, 10988184k reserved, 2886k data, 692k init)
+Memory: 717272k/1049028k available (5831k kernel code, 516k absent, 331240k reserved, 2886k data, 692k init)

[v1: Details added]
[v2: Redid on 32GB box]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>