block/file: be more defensive about the backing file

in file.rs we'll just open the backing file for whatever permissions the disk should have: https://github.com/oxidecomputer/propolis/blob/2dc643742f82d2e072a1281dab23ba2bfdcee440/lib/propolis/src/block/file.rs#L175

but we don't get O_EXCL in here anywhere. I'd thought offhandedly that we could take advantage of that for a very weird one-off test configuration (opening a volume from multiple propolises concurrently), but this was appropriately worrying to everyone else with more calibrated fear-o-meter. we should probably open with O_EXCL, which I think we can plumb in with custom_flags()

if Propolis is told to open a file for writing by multiple VMs concurrently that's likely a serious operator error. I'm less opposed to concurrent read-only opens, but caution seems more appropriate here. I've definitely corrupted guests doing this nonsense locally before.

Start in lib/propolis/src/block/file.rs around line 175 and read how the backing file is opened, then check Rust's Unix OpenOptionsExt::custom_flags documentation. Add the requested exclusive-opening behavior and verify that concurrent writable opens are rejected while the intended read-only behavior remains clear.