portabled: apply pool limits to portable images

portabled: apply pool limits to portable images

Apply SetPoolLimit to IMAGE_PORTABLE instead of IMAGE_MACHINE. The
manager property and the setter now refer to the same portable image
pool.

Reproducer:
  sudo portablectl set-limit 1G
  busctl get-property org.freedesktop.portable1 \
      /org/freedesktop/portable1 \
      org.freedesktop.portable1.Manager PoolLimit

Before:
  SetPoolLimit adjusted the machine image pool while portablectl
  reported the portable image pool. The command could succeed without
  changing the limit that portable users queried later.

Follow-up: 824fcb95c9e66abe6b350ebab6e0593498ff7aa1