vzdump: improve logging output with dirty bitmaps

vzdump: improve logging output with dirty bitmaps

Uses the new 'query-pbs-bitmap-info' QMP call to retrieve additional
information about each drive's dirty bitmap. Returned info is also used
to calculate $target by simply adding all the dirty values (dirty is
equal to size in case the entire drive will be backed up).

"Backup is sparse" message is suppressed for PBS, since it makes little
sense (if zero chunks appear in the clean area of a bitmap, they won't
be counted, and a user is probably more interested in the 'reused' data
anyway).

Also removes the need for the hacky $first_round query-backup handling.

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