ACCL buffers don't support Vitis sw_emu

[Mellich]

Using the XRT driver to create an ACCL buffer for simulation, all create_buffer calls except the one that takes an xrt::bo will create a SimBuffer object that returns a nullptr in the SimBuffer::bo() call.

This becomes a problem, if the buffers created with this call should also be used in a user kernel emulated with Vitis sw_emu.
In this case, an invalid address (nullptr) will be passed to the user kernel and lead to undefined behavior. However, in hardware execution, the same code would work, because another buffer class is used underneath.

This is rather an inconsistency than a bug.