virsh: Null terminated the string memcpy from buffer explicitly

Detected by valgrind:

==16217== 1 errors in context 1 of 12:
==16217== Invalid read of size 1
==16217==    at 0x4A07804: __GI_strlen (mc_replace_strmem.c:284)
==16217==    by 0x3019F167F6: xdr_string (in /lib64/libc-2.12.so)
==16217==    by 0x3033709E8D: xdr_remote_nonnull_string (remote_protocol.c:31)
==16217==    by 0x303370E5CB: xdr_remote_domain_update_device_flags_args (remote_protocol.c:2028)
==16217==    by 0x30337197D1: virNetMessageEncodePayload (virnetmessage.c:341)
==16217==    by 0x30337135E1: virNetClientProgramCall (virnetclientprogram.c:327)
==16217==    by 0x30336F1EFD: callWithFD (remote_driver.c:4586)
==16217==    by 0x30336F1F7B: call (remote_driver.c:4607)
==16217==    by 0x30336F42F2: remoteDomainUpdateDeviceFlags (remote_client_bodies.h:2865)
==16217==    by 0x30336D46E5: virDomainUpdateDeviceFlags (libvirt.c:9457)
==16217==    by 0x41AEE8: cmdChangeMedia (virsh.c:15249)
==16217==    by 0x413CB4: vshCommandRun (virsh.c:18669)
==16217==  Address 0x4ec5e25 is 0 bytes after a block of size 293 alloc'd
==16217==    at 0x4A04A28: calloc (vg_replace_malloc.c:467)
==16217==    by 0x303364F1DB: virAllocN (memory.c:129)
==16217==    by 0x41A844: vshPrepareDiskXML (virsh.c:15043)
==16217==    by 0x41AECC: cmdChangeMedia (virsh.c:15246)
==16217==    by 0x413CB4: vshCommandRun (virsh.c:18669)
==16217==    by 0x423973: main (virsh.c:20261)

tools/virsh.c

@@ -15085,11 +15085,13 @@ cleanup:
     VIR_FREE(device_type);
     VIR_FREE(disk_type);
     if (xml_buf) {
-        if (VIR_ALLOC_N(ret, xmlBufferLength(xml_buf)) < 0) {
+        int len = xmlBufferLength(xml_buf);
+        if (VIR_ALLOC_N(ret, len + 1) < 0) {
             virReportOOMError();
             return NULL;
         }
-        memcpy(ret, (char *)xmlBufferContent(xml_buf), xmlBufferLength(xml_buf));
+        memcpy(ret, (char *)xmlBufferContent(xml_buf), len);
+        ret[len] = '\0';
         xmlBufferFree(xml_buf);
     }
     return ret;