qcow2: Allow resize of images with internal snapshots

qcow2: Allow resize of images with internal snapshots

We originally refused to allow resize of images with internal
snapshots because the v2 image format did not require the tracking of
snapshot size, making it impossible to safely revert to a snapshot
with a different size than the current view of the image.  But the
snapshot size tracking was rectified in v3, and our recent fixes to
qemu-img amend (see 0a85af35) guarantee that we always have a valid
snapshot size.  Thus, we no longer need to artificially limit image
resizes, but it does become one more thing that would prevent a
downgrade back to v2.  And now that we support different-sized
snapshots, it's also easy to fix reverting to a snapshot to apply the
new size.

Upgrade iotest 61 to cover this (we previously had NO coverage of
refusal to resize while snapshots exist).  Note that the amend process
can fail but still have effects: in particular, since we break things
into upgrade, resize, downgrade, a failure during resize does not roll
back changes made during upgrade, nor does failure in downgrade roll
back a resize.  But this situation is pre-existing even without this
patch; and without journaling, the best we could do is minimize the
chance of partial failure by collecting all changes prior to doing any
writes - which adds a lot of complexity but could still fail with EIO.
On the other hand, we are careful that even if we have partial
modification but then fail, the image is left viable (that is, we are
careful to sequence things so that after each successful cluster
write, there may be transient leaked clusters but no corrupt
metadata).  And complicating the code to make it more transaction-like
is not worth the effort: a user can always request multiple 'qemu-img
amend' changing one thing each, if they need finer-grained control
over detecting the first failure than what they get by letting qemu
decide how to sequence multiple changes.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200428192648.749066-3-eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>