Use new dev_open_readonly fn to prevent opening devices for read-write when not neces...

Use new dev_open_readonly fn to prevent opening devices for read-write when not necessary.

Before, we used vg_write_lock_held call to determnine the way a device is
opened. Unfortunately, this opened many devices in RW mode when it was not
really necessary. With the OPTIONS+="watch" rule used in the udev rules,
this could fire numerous events while closing such devices (and it caused
useless scans from within udev rules in return).

A common bug we hit with this was with the lvremove command which was unable
to remove the LV since it was being opened from within the udev rules. This
patch should minimize such situations (at least with respect to LVM handling
of devices).

Though there's still a possibility someone will open a device 'outside' in
parallel and fire the event based on the watch rule when closing a device
once opened for RW.