Fix memory leak in virSCSIDeviceListDel()

Fix memory leak in virSCSIDeviceListDel()

While running virscsitest, it was found that valgrind pointed out the following
memory leak:

==320== 5 bytes in 1 blocks are definitely lost in loss record 4 of 37
==320==    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
==320==    by 0x3E6CE81171: strdup (strdup.c:43)
==320==    by 0x4CB28DF: virStrdup (virstring.c:554)
==320==    by 0x4CAC987: virSCSIDeviceSetUsedBy (virscsi.c:289)
==320==    by 0x402321: test2 (virscsitest.c:100)
==320==    by 0x403231: virtTestRun (testutils.c:199)
==320==    by 0x402121: mymain (virscsitest.c:180)
==320==    by 0x4039AD: virtTestMain (testutils.c:782)
==320==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
==320==

Introduced by commit fd243fc.
Signed-off-by: Ján Tomko <jtomko@redhat.com>