Public api to set/get memory tunables supported by the hypervisors.

dv:
* some cleanups in libvirt.c
* adding extra checks in libvirt.c new entry points

v4:
* Move exporting public API to this patch
* Add unsigned int flags to the public api for future extensions

v3:
* Add domainGetMemoryParamters and NULL in all the driver interface

v2:
* Initialize domainSetMemoryParameters to NULL in all the driver
  interface structure.