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)
    [...]