WmiClass: Don't share "versions" between instances

WmiClass: Don't share "versions" between instances

Lists in Python are mutable and when used as a default value of a
parameter for class constructor, its value will be shared between
all class instances.

Example:

	class Test:
	    def __init__(self, mylist=[]):
	        self.mylist = mylist

	A = Test()
	B = Test()
	A.mylist.append("mylist from instance A")
	print(B.mylist) # Will print ['mylist from instance A']

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>