macvtap: fix a typo

Below patch fixes the following coverity findings

Error: OVERRUN_STATIC:
/libvirt/src/qemu/qemu_command.c:152:
overrun-buffer-val: Overrunning static array "net->mac" of size 6 bytes by passing it as an argument to a function which indexes it at byte position 15.
/libvirt/src/util/virnetdevmacvlan.c:948:
access_dbuff_const: Calling "virNetDevMacVLanVPortProfileRegisterCallback" indexes array "macaddress" at byte position 15.
/libvirt/src/util/virnetdevmacvlan.c:773:
access_dbuff_const: Calling "memcpy" indexes array "macaddress" with index "16UL" at byte position 15.

Error: OVERRUN_STATIC:
/libvirt/src/qemu/qemu_migration.c:2744:
overrun-buffer-val: Overrunning static array "net->mac" of size 6 bytes by passing it as an argument to a function which indexes it at byte position 15.
/libvirt/src/util/virnetdevmacvlan.c:773:
access_dbuff_const: Calling "memcpy" indexes array "macaddress" with index "16UL" at byte position 15.

Error: OVERRUN_STATIC:
/libvirt/src/qemu/qemu_driver.c:435:
overrun-buffer-val: Overrunning static array "net->mac" of size 6 bytes by passing it as an argument to a function which indexes it at byte position 15.
/libvirt/src/util/virnetdevmacvlan.c:1036:
access_dbuff_const: Calling "virNetDevMacVLanVPortProfileRegisterCallback" indexes array "macaddress" at byte position 15.
/libvirt/src/util/virnetdevmacvlan.c:773:
access_dbuff_const: Calling "memcpy" indexes array "macaddress" with index "16UL" at byte position 15.

src/util/virnetdevmacvlan.c

@@ -435,7 +435,7 @@ static const uint32_t modeMap[VIR_NETDEV_MACVLAN_MODE_LAST] = {
 struct virNetlinkCallbackData {
     char *cr_ifname;
     virNetDevVPortProfilePtr virtPortProfile;
-    unsigned char macaddress[VIR_UUID_BUFLEN];
+    unsigned char macaddress[VIR_MAC_BUFLEN];
     char *linkdev;
     int vf;
     unsigned char vmuuid[VIR_UUID_BUFLEN];