lxd/instance/qemu: Don't use the RAM backend
lxd/instance/qemu: Don't use the RAM backend

We were using an inconsistent mix of ram & memfd in some situations
which apparently causes memory issues.

Closes #8400

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

atxwebs/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd