zfsvol: Mark test_xva_export_import as xfail until properly fixed

An issue has been observed while running storage tests:

   tests/storage/storage.py:227: in xva_export_import
       vm: VM | None = source_vm.clone()
   lib/vm.py:663: in clone
      uuid = self.host.xe('vm-clone', {'uuid': self.uuid, 'new-name-label': name})
   lib/host.py:178: in xe
      return self.ssh(command, check=check, simple_output=True)
   lib/host.py:134: in ssh
      return commands.ssh(self.hostname_or_ip, cmd, check=check, simple_output=simple_output,
   E           lib.commands.SSHCommandFailed: SSH command (xe vm-clone   uuid=9b170db4-2914-1bc3-6680-d12539769e6b new-name-label='[jenkins] Alpine minimal for CI_clone_for_tests') failed with return code 1: There was an SR backend failure.
   E           status: Exception
   E           stdout: Only snapshots can be cloned!
   E           stderr: <unknown>

Related-to: XCPNG-3297
Signed-off-by: Philippe Coval <philippe.coval@vates.tech>

Author: rzr

tests/storage/zfsvol/test_zfsvol_sr.py

@@ -93,6 +93,7 @@ def test_invalid_vdi_size(self, zfsvol_sr: SR, image_format: ImageFormat):
             zfsvol_sr.create_vdi(virtual_size=MAX_VDI_SIZE[image_format] + 1)
         assert 'VDI Invalid size' in excinfo.value.stdout
 
+    @pytest.mark.xfail # Failing on Exception "Only snapshots can be cloned!"
     @pytest.mark.small_vm
     @pytest.mark.parametrize("compression", ["none", "gzip", "zstd"])
     def test_xva_export_import(self, vm_on_zfsvol_sr: VM, compression: XVACompression, temp_large_dir: str,