Change VHD implementation to return stream on `.open()` instead of being a stream

[Schamper]

Currently most disk/volume implementations in Dissect have the parsing and the stream reading in the same class. This can cause unintended/unexpected side-effects in various Dissect streams. In an effort to minimize that, we want to change implementations to create a separate stream only when requested.

Basically, going from this:

SomeDisk(fh).read(512)

To this:

some = SomeDisk(fh)
some.open().read(512)

dissect.hypervisor/dissect/hypervisor/disk/vhd.py

Line 23 in 1ef8168

class VHD(AlignedStream):

Needed towards fox-it/dissect.target#100