python: Fix unproxing of VectorParams

python: Fix unproxing of VectorParams

Previously proxy vector parameters would resolve correctly only for
Parent.all. Any other proxy such as Parent.any, or exact ones such as
Parent.addr_range would resolve to a *vector* of the right value
resulting into a vector of a vector. For example if we set:

DirectoryController0.addr_range = [0x100000-0x1fffff, 0x200000-0x2fffff]
DirectoryMemory0.addr_range = Parent.addr_range

where DirectoryController0 is the parent SimObject of DirectoryMemory0
after unproxying the Parent.addr_range VectorParam we would get

DirectoryMemory0.addr_range = [[0x100000-0x1fffff, 0x200000-0x2fffff]]

This change unifies handling of all three proxies to the same correct
unproxy mechanism.

Change-Id: Ie5107f69f58eb700b3e1b92c55210e0d53e6788d
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2901
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>