qemu: Align memory module sizes to 2MiB

qemu: Align memory module sizes to 2MiB

My original implementation was based on a qemu version that still did
not have all the checks in place. Using sizes that would align to odd
megabyte increments will produce the following error:

qemu-kvm: -device pc-dimm,node=0,memdev=memdimm0,id=dimm0: backend memory size must be multiple of 0x200000
qemu-kvm: -device pc-dimm,node=0,memdev=memdimm0,id=dimm0: Device 'pc-dimm' could not be initialized

Introduce an alignment retrieval function for memory devices and use it
to align the devices separately and modify a test case to verify it.