qemu: Extract code to setup memory backing objects

qemu: Extract code to setup memory backing objects

Extract the memory backend device code into a separate function so that
it can be later easily refactored and reused.

Few small changes for future reusability, namely:
- new (currently unused) parameter for user specified page size
- size of the memory is specified in kibibytes, divided up in the
function
- new (currently unused) parameter for user specifed source nodeset
- option to enforce capability check