virtio: add ring buffer type for describing guest memory

Add a ring buffer type that is tailored for holding `struct iovec`
objects that point to guest memory for IO. The `struct iovec` objects
represent the memory that the guest passed to us as `Descriptors` in a
VirtIO queue for performing some I/O operation.

We plan to use this type to describe the guest memory we have available
for doing network RX. This should facilitate us in optimizing the
reception of data from the TAP device using `readv`, thus avoiding a
memory copy.

Co-authored-by: Babis Chalios <[email protected]>
Signed-off-by: Egor Lazarchuk <[email protected]>

Author: bchalios