qapi/qom: Add ObjectOptions for memory-backend-*

This adds a QAPI schema for the properties of the memory-backend-*
objects.

HostMemPolicy has to be moved to an include file that can be used by the
storage daemon, too, because ObjectOptions must be the same in all
binaries if we don't want to compile the whole code multiple times.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>