s390/ipl: sync back loadparm

s390/ipl: sync back loadparm

We expose loadparm as a r/w machine property, but if loadparm is set by
the guest via DIAG 308, we don't update the property. Having a
disconnect between the guest view and the QEMU property is not nice in
itself, but things get even worse for SCSI, where under certain
circumstances (see 789b5a401b "s390: Ensure IPL from SCSI works as
expected" for details) we call s390_gen_initial_iplb() on resets
effectively overwriting the guest/user supplied loadparm with the stale
value.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Fixes: 7104bae9de ("hw/s390x: provide loadparm property for the machine")
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200309133223.100491-1-pasic@linux.ibm.com>
[borntraeger@de.ibm.com: use reverse xmas tree]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>