qemu: Rewrite chardev startup code to use qemuFDPass

qemu: Rewrite chardev startup code to use qemuFDPass

Rewrite the parts which already pass FDs via fdset or directly to use
the new infrastructure.

Apart from simpler code this also adds the appropriate names to the fds
in the fdsets which will allow us to properly remove the fdsets won
hot-unplug of chardevs, which we didn't do for now and resulted in
leaking the FDs.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>