tests.acceptance.avocado_qemu: Add support for powerpc

tests.acceptance.avocado_qemu: Add support for powerpc

Current acceptance test will not run properly in powerpc
environment due qemu target is different from arch, this
usually matches, except with bi-endian architectures like ppc64.
uname would return `ppc64` or `ppc64le` based `big` or `little`
endian but qemu `target` is always `ppc64`. Let's handle it.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Message-Id: <20190819082820.14817-1-sathnaga@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Cleber Rosa <crosa@redhat.com>