hw/arm/spitz: Create SpitzMachineClass abstract base class

For the four Spitz-family machines (akita, borzoi, spitz, terrier)
create a proper abstract class SpitzMachineClass which encapsulates
the common behaviour, rather than having them all derive directly
from TYPE_MACHINE:
 * instead of each machine class setting mc->init to a wrapper
   function which calls spitz_common_init() with parameters,
   put that data in the SpitzMachineClass and make spitz_common_init
   the SpitzMachineClass machine-init function
 * move the settings of mc->block_default_type and
   mc->ignore_memory_transaction_failures into the SpitzMachineClass
   class init rather than repeating them in each machine's class init

(The motivation is that we're going to want to keep some state in
the SpitzMachineState so we can connect GPIOs between devices created
in one sub-function of the machine init to devices created in a
different sub-function.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200628142429.17111-3-peter.maydell@linaro.org