zfsvol: disable coalesce test

vhd-parent, which is used to detect when the coalesce is done, is
not available for that SR type.

Signed-off-by: Gaëtan Lehmann <gaetan.lehmann@vates.tech>

Author: glehmann

tests/storage/zfsvol/test_zfsvol_sr.py

@@ -72,6 +72,7 @@ def test_snapshot(self, vm_on_zfsvol_sr):
 
     @pytest.mark.small_vm
     @pytest.mark.parametrize("vdi_op", ["snapshot"])  # "clone" requires a snapshot
+    @pytest.mark.skip("zfsvol doesn't provide vhd-parent")
     def test_coalesce(
         self, storage_test_vm: VM, vdi_on_zfsvol_sr: VDI, vdi_op: CoalesceOperation, defer: Defer
     ):