When a VFIO device with multiple MMIO regions is hot-unplugged, each
region must be individually matched and removed from the DeviceManager's
mmio_regions list. Compare per-region rather than building an aggregate
across all regions, which would never match any individual entry.
Also remove the now-unused HashSet import.
Signed-off-by: Damian Barabonkov <dbctl@pm.me>