plugins: untaint volume_size_info retuns

plugins: untaint volume_size_info retuns

the size returned by volume_size_info is used for creating the new
destination image in PVE::QemuServer::clone_disk (and probably
elsewhere). In certain cases the return values are tainted - they are
obtained by a run_command call and depending on the format and length
of the parsed output can still have their tainted attribute.

One example of a tainted return has been reported in our
community-forum:
https://forum.proxmox.com/threads/cannot-clone-vm-or-move-disk-with-more-than-13-snapshots.89628/

A qcow2 image with 13 snapshots generates a output > 4k in length from
`qemu-img info --output=json`, which in turn causes the output to be
considered tainted.

This patch untaints the returns where applicable. The other
storage-plugins are not affected:
* LVMPlugin returns a single number and a newline (thus gets untainted
  by run_command)
* RBDPlugin untaints the complete json before decoding
* ZFSPoolplugin and ISCSIDirectPlugin explicitly untaint their
  returns.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>