vzdump: add common log sub-method

vzdump: add common log sub-method

Add a general log method here which supports to pass on the "log to
syslog too" functionality and makes it more clear what each
parameter of logerr and logginfo means.

Further, we can now also log wlith a 'warn' level, which can be
useful to notice an backup user of a possible problem which isn't a
error per se, but may need the users attention.

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