Add support for RAM filesystems for LXC

Add support for RAM filesystems for LXC

Introduce a new syntax for filesystems to allow use of a RAM
filesystem

   <filesystem type='ram'>
      <source usage='10' units='MiB'/>
      <target dir='/mnt'/>
   </filesystem>

The usage units default to KiB to limit consumption of host memory.

* docs/formatdomain.html.in: Document new syntax
* docs/schemas/domaincommon.rng: Add new attributes
* src/conf/domain_conf.c: Parsing/formatting of RAM filesystems
* src/lxc/lxc_container.c: Mounting of RAM filesystems

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>