ui: stdio: replace newlines with whitespaces in prompt messages

ui: stdio: replace newlines with whitespaces in prompt messages

The line-based protocol currently used cannot handle this properly, so
introduce this as a stop-gap measure - otherwise messages might be cut
off.

This makes it work for now, and the text is wrapped correctely for the
screen width in the TUI anyway - which is the only user of this so far.

Will be reworked properly later on.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
 [ TL: add fix-me comment ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>