docs: Add a real-time guide

This beefy patch closes a long-standing TODO and allows us to move yet
more information out of the flavors guide and into specific documents.
This, combined with existing documentation in place, means we can remove
the sections for various extra specs from the 'user/flavors' guide:

- hw:cpu_realtime            -> doc/source/admin/real-time.rst
- hw:cpu_realtime_mask       -> doc/source/admin/real-time.rst
- hw:emulator_threads_policy -> doc/source/admin/cpu-topologies.rst
- hw:cpu_policy              -> doc/source/admin/cpu-topologies.rst
- hw:cpu_thread_policy       -> doc/source/admin/cpu-topologies.rst
- hw:cpu_sockets             -> doc/source/admin/cpu-topologies.rst
- hw:cpu_cores               -> doc/source/admin/cpu-topologies.rst
- hw:cpu_threads             -> doc/source/admin/cpu-topologies.rst
- hw:cpu_max_sockets         -> doc/source/admin/cpu-topologies.rst
- hw:cpu_max_cores           -> doc/source/admin/cpu-topologies.rst
- hw:cpu_max_threads         -> doc/source/admin/cpu-topologies.rst
- hw:numa_nodes              -> doc/source/admin/cpu-topologies.rst
- hw:numa_cpus.N             -> doc/source/admin/cpu-topologies.rst
- hw:numa_mem.N              -> doc/source/admin/cpu-topologies.rst
- hw:mem_page_size           -> doc/source/admin/huge-pages.rst

Multiple improvements to the libvirt extra spec docs are included here,
for want of a better place to include them.

Change-Id: I02b044f8246f4a42481bb5f00259842692b29b71
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin