iSCSI: add support for "backstore_emulate_pr" lrbd setting

oA does not currently expose the advanced option "backstore_emulate_pr", which means to set that one needs to use lrbd directly, which then will get overwritten if anything is done via openATTIC again.

openATTIC should, at the very least, avoid overwriting lrbd configuration that has been added by the user. Even better would be to have SCSI Reservation support configurable via the openATTIC iSCSI UI.