zdtm: More detailed report on unmounting problem

It seems I've screwed the setup and mount points on
my testing machine thus mountpoint test become failing.

Nevertheless, better to report where exactly we've failed
for easier investigation.

With this patch I get

 | ./mountpoints --pidfile=mountpoints.pid --outfile=mountpoints.out
 | 20:26:07.398:  1745: umount(`/dev') failed: Device or resource busy
 | 20:26:07.398:  1745: umount(`/vz') failed: Device or resource busy
 | 20:26:07.399:  1745: umount(`/vz') failed: Device or resource busy
 | 20:26:07.399:  1745: umount(`/vz') failed: Device or resource busy
 | 20:26:07.399:  1745: umount(`/vz') failed: Device or resource busy
 | 20:26:07.399:  1745: FAIL: mountpoints.c:60: Can't umount all the filesystems (errno = 16)
 | 20:26:07.404:  1744: ERR: test.c:298: Test exited with unexpectedly with code 1
 |  (errno = 4)

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>