qemu: fix address allocation on chardev attach

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>