qemu_hotplug: Don't build device string in qemuDomainDetachChrDevice

qemu_hotplug: Don't build device string in qemuDomainDetachChrDevice

This is basically an old artefact from 24b0821926e when the idea
was:

1) Build device string only to see if chardev has any -device
associated with it and thus if device_del is needed
2) Detach chardev using chardev_del

Now, that DEVICE and DEVICE_DELETED capabilities are assumed for
every domain 1) does not make sense anymore.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>