s390x/kvm: factor out SIGP code into sigp.c

We want to use the same code base for TCG, so let's cleanly factor it
out.

The sigp mutex is currently not really needed, as everything is
protected by the iothread mutex. But this could change later, so leave
it in place and initialize it properly from common code.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-17-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>