hw: i386: Use correct RSDT length for checksum

hw: i386: Use correct RSDT length for checksum

AcpiRsdpDescriptor describes revision 2 RSDP table so using sizeof(*rsdp)
for checksum calculation isn't correct since we are adding extra 16 bytes.
But acpi_data_push() zeroes out table, so just by luck we are summing up
exta zeros which still yelds correct checksum.

Fix it up by explicitly stating table size instead of using
pointer arithmetics on stucture.

PS:
Extra 16 bytes are still wasted, but droping them will break migration
for machines older than 2.3 due to size mismatch, for 2.3 and older it's
not an issue since they are using resizable memory regions (a1666142d)
for ACPI blobs. So keep wasting memory to avoid breaking old machines.

Fixes: 72c194f7e (i386: ACPI table generation code from seabios)
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>