storage: Introduce 'extended_l2' feature for storage volume

storage: Introduce 'extended_l2' feature for storage volume

QCOW2 images now support 'extended_l2' which splits the default clusters
into 32 subcluster allocation units. This allows the allocation units to
be smaller without increasing the size of L2 table too much and thus also
the cache requirements for holding the full L2 table in memory.

Unfortunately it's incompatible with qemu versions older than 5.2 thus
can't be used as default.

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