Add support for DC/cluster level 4.8 and qcow2 discard-no-unref flag

[BrooklynDewolf]

Changes introduced with this PR

• Added support for DC/cluster level 4.8

• Added support for the qcow2 discard-no-unref feature

This PR needs https://github.com/oVirt/vdsm/pull/413 in order for hosts to report back CL 4.8.

Are you the owner of the code you are sending in, or do you have permission of the owner?

Yes

[sandrobonazzola]

@shubhaOracle , @dupondje , can you please review?

[sandrobonazzola]

/OST

[sandrobonazzola]

/ost

[sandrobonazzola]

/ost