Update docs for memory parameters and memtune command

Update docs for memory parameters and memtune command

* docs/formatdomain.html.in: Add memtune element details, added min_guarantee
* src/libvirt.c: Update virDomainGetMemoryParameters api description, make
  it more clear that the user first needs to call the api to get the number
  of parameters supported and then call again to get the values.
* tools/virsh.pod: Add usage of new command memtune in virsh manpage