Another exception in Qubes Mabager when shutting down VMs

[rootkovska]

TypeError: float() argument must be a string or a number

This is most likely a bug in the Qubes Manager

----
line: return datetime.datetime.fromtimestamp(float(start_time))
func: get_start_time
line no.: 970
file: /usr/lib64/python2.7/site-packages/qubes/modules/000QubesVm.py
----
line: vm_start_time = vm.get_start_time()
func: check_if_vm_has_shutdown
line no.: 258
file: /usr/lib64/python2.7/site-packages/qubesmanager/main.py

[marmarek]

Duplicate of #2127

[marmarek]

Duplicate of #2127