Small pages memory are not take into account when not explicitly requested

[Sahid Orentino]

Guests using small pages (as default) on compute node are not take into account when calculating available small pages memory [1] - Consequence when booting instance with an explicitly small pages request the compute of available resources is corrupted.

In order to fix the issue two solutions are able.

1/
Associate to every guest a NUMA topology and set the default page_size to MEMPAGES_SMALL when nothing has been requested by user. ** This also implies when using libvirt the default option of virt-type should be KVM **

A small couple of change are needed in hardware.py:
- make the method 'numa_get_constraints' to return NUMATopology in all cases.
- make the method ' _numa_get_pagesize_constraints' to return MEMPAGES_SMALL instead of None when nothing is requested.

2/
Disallow to request a memory page size small, means remove all of the code which handle that case since the information reported to the host are not correctly updated and let the default behavior handle that case.

[1] http://git.openstack.org/cgit/openstack/nova/tree/nova/virt/hardware.py#n1087