[leasefile] lease to xml and back again

On lease acquisition, generate a XML dump of the lease for retrieval
should a supplicant crash and need to be reloaded.
Currently generic lease data and DHCP4 data is supported in both
directions (ie. lease <-> xml) while DHCP6 is currently one way only
(lease -> xml) as lease recovery infra needs to be put in place.
Re-binding of the DHCP4 lease needs investigation as wicked timeout
occurs and retrieved lease state is BOUND:
...
wicked: device eth0 failed: operation timed out
...
2013-12-05T16:12:59.084139-08:00 miranda01 wickedd-dhcp4[31928]: eth0: recovered old lease; now in state=BOUND
...