vzdump: use worker aware log_warn from rest environment for warn level

vzdump: use worker aware log_warn from rest environment for warn level

This ensures that the alert counter is incremented when a message
with such a level is logged, and that the task is prominently marked
in the web UI task log.

The log_warn produces the exact same message format for the warn
level, so we can just swap printing to STDERR for the warning level
without any change to the resulting text in the log. Keep printing to
the (on storage saved) backup log-fd as is.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>