qcow: allow sub-cluster compressed write to last cluster

qcow: allow sub-cluster compressed write to last cluster

Compression in qcow requires image length to be a multiple of the
cluster size.  Lift this requirement by zero-padding the final cluster
when necessary.  The virtual disk size is still not cluster-aligned, so
the guest cannot access the zero sectors.

Note that this is almost identical to the qcow2 version of this code.
qcow2's compression code is drawn from qcow.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>