hw/arm/virt: Disable pl011 clock migration if needed

hw/arm/virt: Disable pl011 clock migration if needed

A clock is added by commit aac63e0e6ea3 ("hw/char/pl011: add a clock
input") since v5.2.0 which corresponds to virt-5.2 machine type. It
causes backwards migration failure from upstream to downstream (v5.1.0)
when the machine type is specified with virt-5.1.

This fixes the issue by following instructions from section "Connecting
subsections to properties" in docs/devel/migration.rst. With this applied,
the PL011 clock is migrated based on the machine type.

   virt-5.2 or newer:  migration
   virt-5.1 or older:  non-migration

Cc: qemu-stable@nongnu.org # v5.2.0+
Fixes: aac63e0e6ea3 ("hw/char/pl011: add a clock input")
Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20210318023801.18287-1-gshan@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>