qemu: fix address allocation on chardev attach
    
    Also check the device type when deciding what type the address should
    be. Commit 9807c47 (aiming to fix another error in address allocation)
    only checked the target type, but its value is different for different
    device types. This resulted in an error when trying to attach
    a channel with target type 'virtio':
    
    error: Failed to attach device from channel-file.xml
    error: internal error: virtio serial device has invalid address type
    
    Make the logic for releasing the address dependent only on
    * the address type
    * whether it was allocated earlier
    to avoid copying the device and target type checks.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1230039
    
    Signed-off-by: Luyao Huang <lhuang@redhat.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>