hw/arm: Set ignore_memory_transaction_failures for most ARM boards

Set the MachineClass flag ignore_memory_transaction_failures
for almost all ARM boards. This means they retain the legacy
behaviour that accesses to unimplemented addresses will RAZ/WI
rather than aborting, when a subsequent commit adds support
for external aborts.

The exceptions are:
 * virt -- we know that guests won't try to prod devices
   that we don't describe in the device tree or ACPI tables
 * mps2 -- this board was written to use unimplemented-device
   for all the ranges with devices we don't yet handle

New boards should not set the flag, but instead be written
like the mps2.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1504626814-23124-3-git-send-email-peter.maydell@linaro.org
For the Xilinx boards:
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>