vzdump: increase timeout for QMP 'cont' after backup start

vzdump: increase timeout for QMP 'cont' after backup start

Since 'backup' can now work asynchronously, QEMU may not be ready to
receive the next QMP command ('cont') immediately. Thus, increase the
timeout, to avoid aborted backups in slow environments.

There may be a deeper QEMU bug hidden under the covers here too, but at
least one user reported success with simply increasing the timeout:
https://forum.proxmox.com/threads/pve7-pbs2-backup-timeout-qmp-command-cont-failed-got-timeout.95212/page-2#post-426261

See also:
https://bugzilla.proxmox.com/show_bug.cgi?id=3693
https://forum.proxmox.com/threads/problem-seit-update-auf-7-0.97388/
https://forum.proxmox.com/threads/error-with-backup-when-backing-up-qmp-command-query-backup-failed-got-wrong-command-id.88017/page-3#post-416339

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