virsh: Don't leak disk targets in cmdDomBlkError

virsh: Don't leak disk targets in cmdDomBlkError

The virDomainGetDiskErrors() API copies disk targets into @disks
array that we allocate. But we forgot to free it:

==140828== 16 bytes in 4 blocks are definitely lost in loss record 41 of 242
==140828==    at 0x4C2F08F: malloc (vg_replace_malloc.c:299)
==140828==    by 0x8C406D9: strdup (in /lib64/libc-2.28.so)
==140828==    by 0x5377DD3: virStrdup (virstring.c:966)
==140828==    by 0x54C112F: testDomainGetDiskErrors (test_driver.c:3068)
==140828==    by 0x55C863D: virDomainGetDiskErrors (libvirt-domain.c:10988)
==140828==    by 0x15D1FA: cmdDomBlkError (virsh-domain-monitor.c:1215)
==140828==    by 0x17F1A8: vshCommandRun (vsh.c:1335)
==140828==    by 0x13489E: main (virsh.c:920)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>