fix #2493: show QEMU errors in migration log

fix #2493: show QEMU errors in migration log

QEMU usually only prints warnings and errors and stays silent otherwise,
so it makes sense to just log all of it's output.

Prefix it with '[<target_hostname>]' to indicate that the output is
coming from the remote node, so users know where to search for the
error.

Side effect is that the 'VM start' task created by the migration will
now show the "QEMU:" prefix, but it's still very readable IMHO.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>