hardware: add method to return requested memory page size

Introduces method to return requested memory pages size.
This commit also add a new exceptions MemoryPageSizeInvalid and
MemoryPageSizeForbidden

Work-Item: Enhance libvirt driver to configure guests based on
           the flavour parameter for page sizes

Partial-Implement: blueprint virt-driver-large-pages
Change-Id: I94e63159ee5539ed7b947614ff0ec87182f184a4
Merge "hardware: add method to return requested memory page size"