libxl: run libxl__arch_domain_create() much earlier.

libxl: run libxl__arch_domain_create() much earlier.

Among other things, arch_domain_create() sets the shadow(/hap/p2m)
memory allocation, which must happen after vcpus are assigned (or the
shadow op will fail) but before memory is allocated (or we might run
out of p2m memory).

libxl__build_pre(), which already sets similar things like maxmem,
semes like a reasonable spot for it.  That needed a bit of plumbing to
get the right datastructure from the caller.

As a side-effect, the return code from libxl__arch_domain_create() is
no longer ignored.

This bug was analysed in:
    From: "Jan Beulich" <JBeulich@xxxxxxxx>
    "Re: [Xen-devel] [xen-unstable test] 16788: regressions - FAIL"
    Date: Mon, 04 Mar 2013 16:34:53 +0000
    http://lists.xen.org/archives/html/xen-devel/2013-03/msg00191.html

Reported-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Tim Deegan <tim@xen.org>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>