Bugs solved:

• Typing regressions caused by incusd/instance/qemu: Fix QMP arguments typing #1531
• Non-hot-unpluggable pre-boot virtiofs shares (Closes: Directory mounts declared before boot cannot be hot-detached on VMs #2323)
• 9p post-boot shares being wrongfully mounted as virtiofs
• Useless checks probably caused by confusion because FSType=9p server-side even in virtiofs shares (note that it’s different agent-side)

Additional actions:

• Remove pre-boot virtiofs logic
• Remove dual virtiofs+9p logic, enforcing a single mount type
• Make the agent a little dumber on path mounts (removing a virtiofs dmesg error when mounting 9p-only shares)

Future directions (to discuss here):

• Maybe add more intelligence to disable virtiofs if image.os and related keys match a house-tailored heuristic?
• Clarify FSType=9p into FSType=share or something like that?
• (more globally) Actually restrict the user / non-internal server operations from setting some volatile keys? (some volatile keys should IMO still be settable by the user, like hwaddr)

That was a tough one to do, I tried to split the commits into easily reviewable ones, although some still are pretty big.