target/s390x/cpu_models: Allow some additional feature bits for the "qemu" CPU

Currently we only present the plain z900 feature bits to the guest,
but QEMU already emulates some additional features (but not all of
the next CPU generation, so we can not use the next CPU level as
default yet). Since newer Linux kernels are checking the feature bits
and refuse to work if a required feature is missing, it would be nice
to have a way to present more of the supported features when we are
running with the "qemu" CPU.
This patch now adds the supported features to the "full_feat" bitmap,
so that additional features can be enabled on the command line now,
for example with:

 qemu-system-s390x -cpu qemu,stfle=true,ldisp=true,eimm=true,stckf=true

Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1495704132-5675-1-git-send-email-thuth@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
