hw/s390x: Build an IPLB for each boot device

hw/s390x: Build an IPLB for each boot device

Build an IPLB for any device with a bootindex (up to a maximum of 8 devices).

The IPLB chain is placed immediately before the BIOS in memory. Because this
is not a fixed address, the location of the next IPLB and number of remaining
boot devices is stored in the QIPL global variable for possible later access by
the guest during IPL.

Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20241020012953.1380075-16-jrossi@linux.ibm.com>
[thuth: Fix endianness problem when accessing the qipl structure]
Signed-off-by: Thomas Huth <thuth@redhat.com>