docs, conf, schema: add support for shmem device

docs, conf, schema: add support for shmem device

This patch adds parsing/formatting code as well as documentation for
shared memory devices.  This will currently be only accessible in QEMU
using it's ivshmem device, but is designed as generic as possible to
allow future expansion for other hypervisors.

In the devices section in the domain XML users may specify:

- For shmem device using a server:

 <shmem name='shmem0'>
   <server path='/tmp/socket-ivshmem0'/>
   <size unit='M'>32</size>
   <msi vectors='32' ioeventfd='on'/>
 </shmem>

- For ivshmem device not using an ivshmem server:

 <shmem name='shmem1'>
   <size unit='M'>32</size>
 </shmem>

Most of the configuration is made optional so it also allows
specifications like:

 <shmem name='shmem1/>
 <shmem name='shmem2'>
   <server/>
 </shmem>

Signed-off-by: Maxime Leroy <maxime.leroy@6wind.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>