hw/s390/ccw.c: Don't take address of packed members

Taking the address of a field in a packed struct is a bad idea, because
it might not be actually aligned enough for that pointer type (and
thus cause a crash on dereference on some host architectures). Newer
versions of clang warn about this.

Avoid the problem by using local copies of the PMCW and SCSW
struct fields in copy_schib_from_guest() and copy_schib_to_guest().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20181213120252.21697-1-peter.maydell@linaro.org>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>