Bug 1736842

+++ This bug was initially created as a clone of Bug #1727347 +++

Description of problem:
The qcow2 spec requires that any v3 file with internal snapshots must use at least 16 bytes of extra data per snapshot entry, in order to include the virtual disk size of the internal snapshot.  However, when converting a v2 file created by older qemu (such as RHEL 6) into a v3 file, the existing snapshot headers are not expanded into the correct v3 format.

True, RHEL tends to frown on internal snapshots (and prefers the use of external), but creating an image that violates the spec, and one that 'qemu-img check' fails to diagnose as violating the spec, is not good for interoperability.

Version-Release number of selected component (if applicable):
qemu-kvm-4.0.0-6.module+el8.1.0+3736+a2aefea3


How reproducible:
100%

Steps to Reproduce:
1. Create a v2 file with internal snapshot. On RHEL6: 
$ qemu-img create -f qcow2 file 1m
$ qemu-img snapshot -c s1 file
2. Check that the internal snapshot header uses the smaller size:
$ od -Ax -j64 -N8 -tx1 file  # Learn the offset for the next command
$ offset=$((0x50000+36))
$ od -Ax -j$offset -N 4 -tx1 file
3. Upgrade it to v3. On RHEL8:
$ qemu-img amend -o compat=1.1 file
4. Check the internal snapshot header size:
$ od -Ax -j64 -N8 -tx1 file  # Learn the offset for the next command
$ offset=$((0x50000+36))
$ od -Ax -j$offset -N 4 -tx1 file
5. Contrast with an image created natively in v3. On RHEL8:
$ qemu-img create -f qcow2 file2 1m
$ qemu-img snapshot -c s1 file2
$ od -Ax -j64 -N8 -tx1 file2  # Learn the offset for the next command
$ offset=$((0x50000+36))
$ od -Ax -j$offset -N 4 -tx1 file2

Actual results:
2. extra size is 0
4. extra size is still 0
5. extra size is 16

Expected results:
4. extra size should change from 0 to 16

Additional info:
qemu-img amend should be taught to rewrite the internal snapshots table to ensure that things get padded out. Having the disk size at the time of each snapshot is a prerequisite to being able to resize an image that contains internal snapshots (we forbid resize of v2 images with internal snapshots because otherwise you may lack the size to switch back to on reverting to that snapshot, but this is a pointless restriction for v3 images).