The image garbage collector should be able to delete images used by VMs

The image garbage collector is crucial for guaranteeing available space for opennebula server.

Sometimes, gcvms will fail to delete image that it considers to be garbage

coin:gcvms.go:256: Destroying:
coin:gcvms.go:258: qtci-linux-openSUSE-15.0-x86_64-9cd6a1
coin:gcvms.go:260: Unable to delete image qtci-linux-openSUSE-15.0-x86_64-9cd6a1: Error placing method call one.image.delete: 2048

because the image is used for running old VM. The logic should be changed so that the image garbage collector will detect if image has running VMs, terminate them and finally delete the image.