ec: Test volume mount point in a better way

An 'ls -a1' on an empty volume seems to return 3 entries instead
of the expected 2 ('.' and '..') in the build servers. I changed
the test to a simple 'stat', which is enough and more reliable.

This is a backport of http://review.gluster.org/8313.

Change-Id: I12d0f47394ad378b40fc9b86507cdb3543f99970
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8313
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8417