Allow specific pool implementation to provide asynchronous
implementation. vm.storage.* methods will detect if given implementation
is synchronous or asynchronous and will act accordingly.
Then it's up to pool implementation how asynchronous should be achieved.
Do not force it using threads (`run_in_executor()`). But pool
implementation is free to use threads, if consider it safe in a
particular case.
This commit does not touch any pool implementation - all of them are
still synchronous.
QubesOS/qubes-issues#2256
Use the right cow image and apply the second layer to provide read-write
access. The correct setup is:
- base image + base cow -> read-only snapshot (base changes "cached"
until committed)
- read-only snapshot + VM cow -> read-write snapshot (changes discarded
after VM shutdown)
This way, even VM without Qubes-specific startup scripts will can
benefit from Template VMs, while VMs with Qubes-specific startup scripts
may still see original root.img content (for possible signature
verification, when storage domain got implemented).
QubesOS/qubes-issues#2256
