qemu: process: Don't leak priv->usbaddrs after VM restart

Since the private data structure is not freed upon stopping a VM, the
usbaddrs pointer would be leaked:

==15388== 136 (16 direct, 120 indirect) bytes in 1 blocks are definitely lost in loss record 893 of 1,019
==15388==    at 0x4C2CF55: calloc (vg_replace_malloc.c:711)
==15388==    by 0x54BF64A: virAlloc (viralloc.c:144)
==15388==    by 0x5547588: virDomainUSBAddressSetCreate (domain_addr.c:1608)
==15388==    by 0x144D38A2: qemuDomainAssignUSBAddresses (qemu_domain_address.c:2458)
==15388==    by 0x144D38A2: qemuDomainAssignAddresses (qemu_domain_address.c:2515)
==15388==    by 0x144ED1E3: qemuProcessPrepareDomain (qemu_process.c:5398)
==15388==    by 0x144F51FF: qemuProcessStart (qemu_process.c:5979)
[...]

(cherry picked from commit 3ab802d6)

src/qemu/qemu_process.c

@@ -6437,6 +6437,10 @@ void qemuProcessStop(virQEMUDriverPtr driver,
     virBitmapFree(priv->autoCpuset);
     priv->autoCpuset = NULL;
 
+    /* remove address data */
+    virDomainUSBAddressSetFree(priv->usbaddrs);
+    priv->usbaddrs = NULL;
+
     /* The "release" hook cleans up additional resources */
     if (virHookPresent(VIR_HOOK_DRIVER_QEMU)) {
         char *xml = qemuDomainDefFormatXML(driver, vm->def, 0);