vmspawn-qmp: derive QMP node and device ids from a bridge counter

vmspawn-qmp: derive QMP node and device ids from a bridge counter

Replace the caller-supplied DriveInfo.node_name with two QMP-internal
strings generated at setup time from a monotonic per-bridge counter:

  qmp_node_name   = "vmspawn-<N>-storage"  (blockdev node-name)
  qmp_device_id   = "vmspawn-<N>-disk"     (qdev id)

This is the naming scheme the upcoming runtime-hotplug add path needs:
unique across the lifetime of the VM, decoupled from any user-visible id,
and stable across the four QMP commands that make up an add (add-fd,
blockdev-add, remove-fd, device_add). The boot-time setups don't care
about uniqueness, but switching them now means the hotplug path can share
qmp_build_device_add() and EphemeralDriveCtx without a parallel naming
scheme.

vmspawn.c stops assigning node_name in prepare_primary_drive (which used
the literal "vmspawn") and prepare_extra_drives (which counted
"vmspawn_extra_%zu"); both are replaced by the bridge counter at the
point the drive is actually pushed into QEMU. Ephemeral helper-node
names follow the same vmspawn-<N>-{base-file,base-fmt,overlay-file}
convention; the blockdev-create job-id becomes
vmspawn-<N>-overlay-create.

EphemeralDriveCtx grows a qmp_device_id field (the qdev id is
independent of the format node-name now) and renames node_name →
qmp_node_name to match. No external behaviour change other than the new
internal names.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>