tests/acceptance: introduce arch parameter and attribute

tests/acceptance: introduce arch parameter and attribute

It's useful to define the architecture that should be used in
situations such as:
 * the intended target of the QEMU binary to be used on tests
 * the architecture of code to be run within the QEMU binary, such
   as a kernel image or a full blown guest OS image

This commit introduces both a test parameter and a test instance
attribute, that will contain such a value.

Now, when the "arch" test parameter is given, it will influence the
selection of the default QEMU binary, if one is not given explicitly
by means of the "qemu_img" parameter.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-5-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>