qemuxml2argvtest: Make 'qemu-ns' test case more stable

qemuxml2argvtest: Make 'qemu-ns' test case more stable

The test was showing that the 'blockdev' capability is properly added
although we didn't detect it yet. Unfortunately this test can't be
carried over once we bump minimum qemu version to qemu-4.2.

Make the test case future-proof by removing the qemu-4.0.0 version which
would become pointless and use only already deprecated capability flags
so that the test output does not change.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>