virtq: async support

[cagatay-y]

Adds a proof-of-concept async implementation to virtq and uses it in the file system device driver.

Stuff I need to investigate before the PR is ready:

• Can async packet transmission / reception be used for the smoltcp based networking stack?
• What is the correct mutex type for the use case in the PR? I chose pretty much at random (except for avoiding the interrupt disabling variant. I am not sure if it would affect the kind of interrupts we rely on for virtq notifications).
• Why does the code not work without the dbg statement in VirtioFsDriver::send_command?
• Is the current block_on implementation suitable for this use case? There is some stuff in there that is related to the network drivers. At the very least, the return error type needs to be made more general to allow VirtqErrors.

Ideas for future additions:

• Support for async dispatch functions that await on used notifications to remove them when they are done and use the newly open space to transmit new packets. This would be useful in queues where the used buffer is not useful to the driver (i.e. TX queues).