Deprecate KVM support for AArch32

The Linux kernel has dropped support for allowing 32-bit Arm systems
to host KVM guests (kernel commit 541ad0150ca4aa663a2, which just
landed upstream in the 5.7 merge window).  Mark QEMU's support for
this configuration as deprecated, so that we can delete that support
code in 5.2.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>

docs/system/deprecated.rst

@@ -336,6 +336,14 @@ The ``compat`` property used to set backwards compatibility modes for
 the processor has been deprecated. The ``max-cpu-compat`` property of
 the ``pseries`` machine type should be used instead.
 
+KVM guest support on 32-bit Arm hosts (since 5.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''
+
+The Linux kernel has dropped support for allowing 32-bit Arm systems
+to host KVM guests as of the 5.7 kernel. Accordingly, QEMU is deprecating
+its support for this configuration and will remove it in a future version.
+Running 32-bit guests on a 64-bit Arm host remains supported.
+
 System emulator devices
 -----------------------