Log Message:
emulators/qemu: New option qemu-virtfs.

Add NetBSD support for qemu-virtfs.  (And pave the way for other BSD
support too.)

Narrow the existing virtfs-proxy-helper option to use
--enable-virtfs-proxy-helper.  I doubt this option has worked in a
long time, because I think virtfs-proxy-helper has been off by
default even with virtfs enabled; maybe we should just delete it.

No functional change intended for default build on non-NetBSD
platforms.

PR pkg/59163: emulators/qemu: missing virtfs option for NetBSD