fix #4556: introduce 'dc' and 'vm' prefix for IPSets

fix #4556: introduce 'dc' and 'vm' prefix for IPSets

to differentiate whether they should be taken from the datacenter config
or from the local config. The parser now accepts IPSets in the following
format:

    +dc/ipset
	Looks for the IPSet on the Datacenter level.
    +vm/ipset
	Looks for the IPSet on the VM level.
    +ipset
	Uses the previous method of scoping, where it first looks at the
	VM level and then at the Datacenter level.

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>