xlators: Add Block Device(BD) backend translator

Add a new server storage xlator 'bd mapper'. Intention of this xlator is
to add block device backend support to gluster. It exports block devices
as regular files to the gluster client.

The immediate goal of this translator is to use logical volumes to
store VM images and expose them as files to QEMU/KVM.

Given Volume group is represented as directory and its logical
volumes as files.

By exporting LUNs/LVs as regular files, it becomes possible to:
* Associate each VM to a LV/LUN
* Use file system commands like cp to take copy of VM images
* Create linked clones of VM  by doing LV snapshot at server
side
* Implement thin provisioning by developing a qcow2 translator

As of now this patchset maps only logical volumes. BD Mapper volume
file specifies which Volume group to export to the client. BD xlator
exports the volume group as a directory and all logical volumes under that
as regular files.

BD xlator uses lvm2-devel APIs for getting the list of Volume Groups
and Logical Volumes in the system.

The eventual goal of this work is to support thin provisioning,
snapshot, copy etc of VM images seamlessly in glusterfs storage environment

BUG: 805138
Change-Id: I13b69d39d7fd199c101c8e9e4f2cf10772bdc3dd
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-on: http://review.gluster.org/3551
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>

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