qemu_command: Generate memory only after controllers

qemu_command: Generate memory only after controllers

Currently, memory device (def->mems) part of cmd line is
generated before any controller. In majority of cases it doesn't
matter because neither of memory devices live on a bus that's
created by an exposed controller (e.g. there's no DIMM
controller, at least not exposed). Except for virtio-mem and
virtio-pmem, which do have a PCI address. And if it so happens
that the device goes onto non-default bus (pci.0) starting such
guest fails, because the controller that creates the desired bus
wasn't processed yet. QEMU processes arguments in order.

For instance, if virtio-mem has address with bus='0x01' QEMU
refuses to start with the following message:

  Bus 'pci.1' not found

Similarly for virtio-pmem. I've successfully tested migration and
changing the order does not affect migration stream.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2047271
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>