Regression test portability: loopback devices

Introduce functions to deal with loopback devices setup, mount
and umount.

Remove test for xfsprogs for non Linux systems, as loopback devices
can be populated with other filesystems (e.g.: FFS for NetBSD)

While there, remove mount.nfs test for non Linux systems. At least
NetBSD has it in base system as mount_nfs.

BUG: 1129939
Change-Id: I816b36e1d3e6933f92acf19d9be8eeaaa333356e
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8914
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>