configure: Add --enable-migration-from-qemu-kvm

This switch will turn on all the migration compat bits needed to
perform migration from qemu-kvm to qemu. It's just a stub for now.

This compat will break incoming migration from qemu < 1.3, but for
distros where qemu-kvm was the only shipped package for years it's
not a big loss (and I don't know any way to avoid it).
acpi_piix4: Drop minimum_version_id to handle qemu-kvm migration

qemu-kvm 1.2 advertised version_id=2, but it was not the same
format as qemu.git version_id=2.

commit b0b873a07872f7ab7f66f259c73fb9dd42aa66a9 added the qemu-kvm
format to qemu.git, but was forced to call it version_id=3, and
bumped minimum_version_id to 3. This breaks incoming migration from
qemu-kvm.

If --enable-migration-from-qemu-kvm is enabled, drop minimum_version_id
to 2. Migration from qemu-kvm version_id=2 and qemu 1.3+ version_id=3
works, but migration from qemu < 1.3 is broken.
i8254: Fix migration from qemu-kvm < 1.1

qemu-kvm commit 81bdec908fb2be0ccaff1d4ee67956c509e440ad did this,
but the logic can't be carried unconditionally in qemu.git without
breaking migration from qemu < 1.1.

Conditionalize it with --enable-migrate-from-qemu-kvm
pc_piix: Add compat handling for qemu-kvm VGA mem size

Paolo outlines this here:

https://lists.gnu.org/archive/html/qemu-devel/2013-01/msg02540.html

qemu-kvm defaulted to vgamem=16MB since at least 0.15, while qemu used
8MB. For qemu 1.2, the default was changed to 16MB for all devices
except cirrus.

If --enable-migration-from-qemu-kvm is specified, make sure cirrus
uses 16MB for <= pc-1.2 (the qemu-kvm merge), and 16MB always for
all others. This will break incoming qemu migration for qemu < 1.3.