Add documentation for loading QCOW2 snapshots

Add a section on how to use QCOW2 module to load
images that are based on backing files.

This is related to changes on: fox-it/dissect.hypervisor#64

Signed-off-by: Andreia Ocanoaia <andreia.ocanoaia@gmail.com>

Author: andreia-oca

docs/source/projects/dissect.hypervisor/index.rst

@@ -57,6 +57,49 @@ a VMDK for reading:
 Many of the parsers in this package behave in a very similar way, so check the API reference to see how to utilize the
 parser you need.
 
+Open QCOW2 snapshots
+~~~~~~~~~~~~~~~~~~~~
+
+For `qcow2` images there is support for backing-files and it can either be automatically loaded when opening a target.
+The backing-file will automatically be read from the `qcow2` headers and dissect will attempt to load it.
+
+.. code-block:: python
+  target = Target.open(target_path)
+  print(target.users())
+
+Or, for more control, the path to the backing file can be passed when initializing a `qcow2` disk:
+
+.. code-block:: python
+  def open_qcow2_with_backing_file(snapshot_path: Path, backing_path: Path):
+      # Open base QCOW2 image
+      backing_fh = backing_path.open("rb")
+      base_qcow2 = qcow2.QCow2(backing_fh)
+      base_stream = base_qcow2.open()
+
+      # Open snapshot QCOW2 image with base as backing file
+      snapshot_fh = snapshot_path.open("rb")
+      snapshot_qcow2 = qcow2.QCow2(
+          snapshot_fh,
+          backing_file=base_stream
+      )
+      snapshot_stream = snapshot_qcow2.open()
+
+      return snapshot_stream, snapshot_fh, backing_fh, base_stream
+
+  def analyze_image(snapshot_path: Path, backing_path: Path):
+      # Open the QCOW2 snapshot along with its backing file and get file/stream handles
+      snapshot_stream, snapshot_fh, backing_fh, base_stream = open_qcow2_with_backing_file(snapshot_path, backing_path)
+
+      # Create a new Dissect target to analyze the disk image
+      target = Target()
+      # Add the snapshot stream to the target’s disks
+      target.disks.add(snapshot_stream)
+      # Resolve all disks, volumes and filesystems and load an operating system on the current
+      target.apply()
+
+      # Collect data from the snapshot
+      print(target.users())
+
 Tools
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