target: use new "dbroot" target attribute

This commit updates the target core ALUA and PR
modules to use the new "dbroot" attribute instead
of assuming the target database is in "/var/target".

Signed-off-by: Lee Duncan <lduncan@suse.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>