Fix qcow2 support for absolute backing_file paths

_open_backing_file assumed that the backing_file path
was always a filename relative to the snapshot path.
Using .with_name() to concat the whole path broke the use
case for absolute path for the backing_file causing
ValueError("Invalid name ...").

This change updates the logic to detect whether auto_backing_file is an
absolute path. If so, it is used directly; otherwise we fall back to
constructing the path relative to the snapshot location.

This will support the following use-cases:
backing file: /home/user/workdir/ubuntu-22.04.qcow2
backing file: ubuntu-22.04.qcow2
backing file: ./ubuntu-22.04-packer.qcow2
backing file: ../../ubuntu-22.04-packer.qcow2

Fixes: #64

Signed-off-by: Andreia Ocanoaia <[email protected]>

Author: andreia-oca

dissect/hypervisor/disk/qcow2.py

@@ -188,7 +188,13 @@ def _open_backing_file(self, backing_file: BinaryIO | None, allow_no_backing_fil
         backing_file_path = None
         if backing_file is None:
             if self.path:
-                if (backing_file_path := self.path.with_name(self.auto_backing_file)).exists():
+                auto_backing_path = Path(self.auto_backing_file)
+                backing_file_path = (
+                    auto_backing_path
+                    if auto_backing_path.is_absolute()
+                    else self.path.with_name(auto_backing_path.name)
+                )
+                if backing_file_path.exists():
                     backing_file = backing_file_path.open("rb")
                 elif not allow_no_backing_file:
                     raise Error(