fixed issue in yardstick-img-modify cleanup() fun

fixed issue in yardstick-img-modify cleanup() fun

1. when run yardstick-img-modify script, sometimes it show below error.
 + '[' -f /tmp/workspace/yardstick/yardstick-xenial-server.raw ']'
 + kpartx -dv /tmp/workspace/yardstick/yardstick-xenial-server.raw
 ioctl: LOOP_CLR_FD: No such device or address
 del devmap : loop0p1
 can't del loop : /dev/loop0
 + test 1 -eq 1 -o 1 -eq 2
 2. so i added a expression to check if the .raw file map deleted or not
 3. if the devmap not deleted, then we delete it, otherwise, we do nothing

JIRA: YARDSTICK-502

Change-Id: Ia050121ed6bf837bab05e38f2e9e9628ca31627a
Signed-off-by: Ryan.RCS <lihainong@huawei.com>