Return -1 in virPortAllocatorAcquire if all ports are used
    
    Report the error in virPortAllocatorAcquire instead
    of doing it in every caller.
    
    The error contains the port range name instead of the intended
    use for the port, e.g.:
    Unable to find an unused port in range 'display' (65534-65535)
    instead of:
    Unable to find an unused port for SPICE
    
    This also adds error reporting when the QEMU driver could not
    find an unused port for VNC, VNC WebSockets or NBD migration.