qemu: fd: Add a distinct set of APIs for 'direct' fd passing mode

qemu: fd: Add a distinct set of APIs for 'direct' fd passing mode

Originally I envisioned a common set of APIs for both FD passing
approaches but it turns out they are not really compatible enough for it
to make sense to use one set of APIs.

As of such introduce a distinct set of APIs for the 'direct' mode, which
will later be used to convert all places that currently use
'qemuFDPassNewDirect' and later clean up the existing APIs.

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