vrunner: poll for clean VM exit before SIGTERM in daemon_stop

The daemon_stop() flow sent ===SHUTDOWN=== over virtio-serial and then
slept exactly 2 seconds before unconditionally SIGTERMing QEMU. The
guest's graceful_shutdown(), which is what the SHUTDOWN command
ultimately triggers, does:

  sync
  umount /var/lib/containers/storage   (ext4 journal commit)
  sync
  blockdev --flushbufs (per disk)
  sync
  sleep 2
  reboot -f

Under load this routinely takes 5-30 seconds — the ext4 journal commit
on the state disk after a vimport that just wrote tens of MB of layer
blobs is the slow step. A fixed 2-second wait followed by SIGTERM kills
the guest mid-umount and leaves the state disk's journal half-committed:
layer files have correct inode metadata but partially-unwritten data
extents.

The next memres session remounts that disk and reads the apparently-OK
files. Tar-split reassembly during podman save / podman push then hits
the unwritten extents and produces:

  Error: reading blob sha256:<hash>: EOF
  Error: reading blob sha256:<hash>: file integrity checksum failed
         for "<file>"

Reported via yocto-patches as a workaround in run-push-containers
(`<runtime> image rm --all` before push) on the autobuilder. The
"first push works, subsequent fail" pattern in that report comes from
the first push hitting a clean session and subsequent pushes inheriting
the corrupted state disk from the prior SIGTERM-truncated shutdown.

Replace the fixed sleep with a poll loop that watches for the QEMU
process to exit, up to 60 seconds (120 * 0.5s). 60s is generous enough
to cover heavy ext4 journal commits; short enough that a truly hung
guest doesn't block the caller indefinitely. The existing SIGTERM and
SIGKILL escalation paths remain as fallbacks for the genuinely-stuck
case.

Reproducer: vimport an OCI image as testimg:latest, save it, memres
restart, re-vimport, save again. Without this fix the second save
fails 100% of the time on the same blob digest with EOF or CRC error.
With this fix six consecutive vimport+save cycles across three restart
rounds complete cleanly.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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