Fix dirty-bitmap PBS backup with multiple drives

Fix dirty-bitmap PBS backup with multiple drives

"PVE backup: rename incremental to use-dirty-bitmap" merged two
variables (use_dirty_bitmap and incremental) into one, but they served
two different purposes. Rename the original use_dirty_bitmap to
"expect_only_dirty" so the new one doesn't conflict, and rework "PVE:
use proxmox_backup_check_incremental" around that semantic.

In practice, this had the effect that only one disk at a time would
have a bitmap added, as after the first "use_dirty_bitmap" would be set
to one and the rest would behave as if the QMP parameter of the same
name was unset.

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