Description of problem

In kata-containers/runtime#624, we have such problems:

1. Hot add memory should be aligned to memory section size, but the size of a memory section is architecture dependent. For example, power uses 16MB, ia64 uses 1GB, x86_64 uses 128MB, ppc64le uses 256MB.
2. Each memory block is described under /sys/devices/system/memory as /sys/devices/system/memory/memoryXXX (XXX is the memory block id.), which is the unit of memory online/offline. In sparse memory model, memory block size is a multiple of memory section size, reference. We can online a memory block only if we hotplug a memory-block-sized dimm, see pages_correctly_reserved in reference.

So we should get the size of guestos memory block by reading /sys/devices/system/memoryblock_size_bytes in sandbox, and make hot add memory to be aligned to memory block size of sandbox.

I think it's needed to add a ReadSandboxFile rpc to read file above in a common way.