Improve bhyve mevent EVF_VNODE monitoring
#14081 introduced a feature allowing notification callbacks when the attributes of a file changed and this is used to inform some drivers of a size change - initially this was only available for virtio block devices but work is in progress to allow for a size change to trigger an AEN on nvme too.
When this was ported to illumos, the attribute monitoring was implemented via event ports. However, this only currently works for file-backed storage devices and not those utilising ZFS volumes, since those do not support event ports.
We should add a fallback mechanism to watch for changes in the size of a ZVOL that underlies a storage backend so that size change notifications work properly.
This will also remove the warning that is currently output by bhyve when booting a VM that uses a ZVOL.