In the context of the RHEV management application, iSCSI/SCSI Fibre are
providing the raw storage, with LVM VGs on top and the carving LVs for
the guests. In the common case the admin/app would monitor VG usage& LV
rate of increase to ensure extra space was available in the VG ahead of
it being needed. eg if the VG comes close to exhaustion then further LUNS
can be obtained and added as PVs to the LVM volume group. So you can't
guarentee that a VM won't stop on ENOSPC, but it is very unlikely if the
system is operating correctly.
As an added complication, since cluster-LVM isn't used, all LVM operations
have to be performed on a dedicated/exclusive storage host and then metadata
refreshed/propagated to other hosts running VMs. This last issue implies that
letting QEMU resize its LV would never be possible, even if it were not for
the permissions problem.