iotests: Disable 125 on broken XFS versions

iotests: Disable 125 on broken XFS versions

And by that I mean all XFS versions, as far as I can tell.  All details
are in the comment below.

We never noticed this problem because we only read the first number from
qemu-img info's "disk size" output -- and that is effectively useless,
because qemu-img prints a human-readable value (which generally includes
a decimal point).  That will be fixed in the next patch.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190925183231.11196-3-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>