extend save/restore on KVM VMM_MAD to have both pre and post hooks

[atodorov-storpool]

Description
Brief description of the new functionality
Extend VMM_MAD save/restore to call tm_mad pre and post scripts

Use case
On System datastores with TM_MAD backed by block devices it is possible to store the checkpoint file directly on block device, skipping the step of storing the file on Host filesystem first.

Interface Changes
Extend save and restore on VMM_MAD to have both "pre" and "post" hooks to SYSTEM DS's TM_MAD. With the change the vmm/save script will look for vmm/save.<tm_mad>-pre and vmm/restore will look for vmm/restore.<tm_mad>-post. Both pieces of code are borrowed from the existing parts of current save/restore

Additional Context
Following the forum discussion

I think that the ceph TM_MAD could be updated to use this functionality but have no ceph cluster to test it, though.

Pull request #3270

There is alternative solution by @kvaps introduced in #3272

Progress Status

• Branch created
• Code committed to development branch
• Testing - QA
• Documentation
• Release notes - resolved issues, compatibility, known issues
• Code committed to upstream release/hotfix branches
• Documentation committed to upstream release/hotfix branches

[kvaps]

link #3272

[atodorov-storpool]

I am closing #3270 and #3271 in favour of #3272 because this solution could be implemented faster and at the moment with a configuration change that does not violate the OpenNebula Polices.