qtest: ask endianness of the target in qtest_init()

qtest: ask endianness of the target in qtest_init()

The target endianness is not deduced anymore from
the architecture name but asked directly to the guest,
using a new qtest command: "endianness". As it can't
change (this is the value of TARGET_WORDS_BIGENDIAN),
we store it to not have to ask every time we want to
know if we have to byte-swap a value.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
CC: Greg Kurz <groug@kaod.org>
CC: David Gibson <david@gibson.dropbear.id.au>
CC: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>