qemuProcessPrepareHostBackendChardevFileHelper: Always use FD passing

qemuProcessPrepareHostBackendChardevFileHelper: Always use FD passing

Code paths which don't wish to use FD passing are supposed to not call
the function which sets up the chardev for FD passing.

This is ensured by calling it only in the host prepare step.

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