tools/libxl: Reposition build_pre() logic between architectures

tools/libxl: Reposition build_pre() logic between architectures

The call to xc_domain_disable_migrate() is made only from x86, while its
handling in Xen is common.  Move it to the libxl__build_pre().

hvm_set_conf_params(), hvm_set_viridian_features(),
hvm_set_mca_capabilities(), and the altp2m logic is all in common code (parts
ifdef'd) but despite this, is all actually x86 specific, as least as currently
implemented in Xen.  Some concepts (nested virt, altp2m) are common in
principle, but need their interface changing to be part of domain_create, and
are not expecting to survive in their current HVM_PARAM form.

Move it all into x86 specific code, and fold all of the xc_hvm_param_set()
calls together into hvm_set_conf_params() in a far more coherent way.

Finally - ensure that all hypercalls have their return values checked.

No practical change in constructed domains.  Fewer useless hypercalls now to
construct an ARM guest.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>