pc: hack for migration compatibility from QEMU 2.0

Changing the ACPI table size causes migration to break, and the memory
hotplug work opened our eyes on how horribly we were breaking things in
2.0 already.

The ACPI table size is rounded to the next 4k, which one would think
gives some headroom.  In practice this is not the case, because the user
can control the ACPI table size (each CPU adds 97 bytes to the SSDT and
8 to the MADT) and so some "-smp" values will break the 4k boundary and
fail to migrate.  Similarly, PCI bridges add ~1870 bytes to the SSDT.

This patch concerns itself with fixing migration from QEMU 2.0.  It
computes the payload size of QEMU 2.0 and always uses that one.
The previous patch shrunk the ACPI tables enough that the QEMU 2.0 size
should always be enough; non-AML tables can change depending on the
configuration (especially MADT, SRAT, HPET) but they remain the same
between QEMU 2.0 and 2.1, so we only compute our padding based on the
sizes of the SSDT and DSDT.

Migration from QEMU 1.7 should work for guests that have a number of CPUs
other than 12, 13, 14, 54, 55, 56, 97, 98, 139, 140.  It was already
broken from QEMU 1.7 to QEMU 2.0 in the same way, though.

Even with this patch, QEMU 1.7 and 2.0 have two different ideas of
"-M pc-i440fx-2.0" when there are PCI bridges.  Igor sent a patch to
adopt the QEMU 1.7 definition.  I think distributions should apply
it if they move directly from QEMU 1.7 to 2.1+ without ever packaging
version 2.0.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
