Extend VirtioBlkConfig enough to cover up to write_zeroes_may_unmap, as
we're going to need those fields for implementing support both
F_DISCARD and F_WRITE_ZEROES.

Signed-off-by: Sergio Lopez <[email protected]>

Imago's discard_to_* operations require a mutable reference to Storage.
Let's wrap disk_image in a Mutex so we can obtain such reference when
needed. Since, in practice, we won't have multiple threads accessing the
lock, the overhead should be very small (one or two operations on
atomics).

Signed-off-by: Sergio Lopez <[email protected]>

The F_DISCARD feature enables the guest to request us to free up sectors
on the backing storage. Since imago does most of the heavy lifting, we
just need to announce the feature and call to imago's "discard_to_any"
then needed.

Signed-off-by: Sergio Lopez <[email protected]>

The F_WRITE_ZEROES feature enables the guest to instruct the host to
write zeroes to the disk image without actually having to copy any data
around. Since imago does the heavy lifting for us, we just need to
announce the feature and call to the right method when needed.

Signed-off-by: Sergio Lopez <[email protected]>