hw/s390/css: avoid taking address members in packed structs

The GCC 9 compiler complains about many places in s390 code
that take the address of members of the 'struct SCHIB' which
is marked packed:

hw/s390x/css.c: In function ‘sch_handle_clear_func’:
hw/s390x/css.c:698:15: warning: taking address of packed member of ‘struct SCHIB’ may result in an unaligned pointer val\
ue [-Waddress-of-packed-member]
  698 |     PMCW *p = &sch->curr_status.pmcw;
      |               ^~~~~~~~~~~~~~~~~~~~~~
hw/s390x/css.c:699:15: warning: taking address of packed member of ‘struct SCHIB’ may result in an unaligned pointer val\
ue [-Waddress-of-packed-member]
  699 |     SCSW *s = &sch->curr_status.scsw;
      |               ^~~~~~~~~~~~~~~~~~~~~~

...snip many more...

Almost all of these are just done for convenience to avoid
typing out long variable/field names when referencing struct
members. We can get most of this convenience by taking the
address of the 'struct SCHIB' instead, avoiding triggering
the compiler warnings.

In a couple of places we copy via a local variable which is
a technique already applied elsewhere in s390 code for this
problem.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190329111104.17223-13-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>