If a management application (like Libvirt) want's to preserve
migration ability and switch to '-machine memory-backend' it
needs to set exactly the same RAM id as QEMU would. Since the id
is machine type dependant, expose it under 'query-machines'
result. Some machine types don't have the attribute set (riscv
family for example), therefore the QMP attribute must be
optional.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <9384422f63fe594a54d801f9cb4539b1d2ce9b67.1590481402.git.mprivozn@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
[ehabkost: updated doc to "since 5.2"]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>