Description of problem:
Revert "qemu-block: deprecated/defunct, remove from tree"

qemu-block xlator addesses the file snapshots for the block store
requirements.

This xlator exposes a qcow2 file as a raw file for client, also play
a key role in achieving file snapshots feature  in gluster
(qcow2 based) both internal and external.

Someone like a tcmu-runner (service which exposes a file in userspace
as a iSCSI block backend, intern taking support of tcmu kernel module)
who can communicate with gluster using gfapi can take qemu-block
xlator advantage.


Version-Release number of selected component (if applicable):
mainline