build: disable Xen on ARM

While ARM could present the xenpv machine, it does not and trying to enable
it breaks compilation.  Revert to the previous test which only looked at
$target_name, not $cpu.

Fixes: 3b6b7550
Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170711100049.20513-1-pbonzini@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>