util: usb: define cleanup function using VIR_DEFINE_AUTOPTR_FUNC
    
    Using the new VIR_DEFINE_AUTOPTR_FUNC macro defined in
    src/util/viralloc.h, define a new wrapper around an existing
    cleanup function which will be called when a variable declared
    with VIR_AUTOPTR macro goes out of scope. Also, drop the redundant
    viralloc.h include, since that has moved from the source module into
    the header.
    
    When a variable of type virUSBDevicePtr is declared using
    VIR_AUTOPTR, the function virUSBDeviceFree will be run
    automatically on it when it goes out of scope.
    
    Signed-off-by: Sukrit Bhatnagar <skrtbhtngr@gmail.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>