examples/vhost_blk: fix check of device path

examples/vhost_blk: fix check of device path

Fix the time of check time of use warning in example code.
Ignore the errno of unlink failure. There are two situations.
The first one is that file doesn't exist the unlink fails and
it's ok to ignore. The second one is that unlink fails to remove
file but the next bind() would fail too.

Coverity issue: 350589, 158663
Fixes: c19beb3f38cd ("examples/vhost_blk: introduce vhost storage sample")
Cc: stable@dpdk.org
Signed-off-by: Jin Yu <jin.yu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>