qdev: Implement (variable length) array properties

qdev: Implement (variable length) array properties

Add support for declaring array properties for qdev devices.
These work by defining an initial static property 'len-arrayname'
which the user of the device should set to the desired size
of the array. When this property is set, memory is allocated
for the array elements, and dynamic properties "arrayname[0]",
"arrayname[1]"... are created so the user of the device can
then set the values of the individual array elements.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>