When parsing the guest XML we must fill in the default guest arch if it
is not already present because later parts of the parsing process need
this information.

If no arch is specified we lookup the first guest in the capabilities
data matching the os type and virt type. In most cases this will result
in picking the host architecture but there are some exceptions...

 - The test driver is hardcoded to always use i686 arch
 - The VMWare/ESX drivers will always place i686 guests ahead
   of x86_64 guests in capabilities, so effectively they always
   use i686
 - The QEMU driver can potentially return any arch at all
   depending on what combination of QEMU binaries are installed.

The domain XML hardware configurations are inherently architecture
specific in many places. As a result whomever/whatever created the
domain XML will have had a particular architecture in mind when
specifying the config. In pretty much any sensible case this arch
will have been the native host architecture. i686 on x86_64 is
the only sensible divergance because both these archs are
compatible from a domaain XML config POV.

IOW, although the QEMU driver can pick an almost arbitrary arch as its
default, in the real world no application or user is likely to be
relying on this default arch being anything other than native.

With all this in mind, it is reasonable to change the XML parser to
allow the default architecture to be passed via the domain XML options
struct. If no info is explicitly given then it is safe & sane to pick
the host native architecture as the default for the guest.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>