vfio: Move vfio_reset_handler() into device.c

vfio: Move vfio_reset_handler() into device.c

Pass-through devices of a VM are not necessarily in the same group and
all groups/address_spaces need to be scanned when the machine is
reset. Commit f16f39c3fc97 ("Implement PCI hot reset") introduced a VM
reset handler for this purpose. Move it under device.c

Also reintroduce the comment which explained the context and was lost
along the way.

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-26-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>