Support virtio-fs on MMIO

[hcsch]

The non-transport specific parts of the VIRTIO filesystem driver are feature gated on pci, even though they are not transport specific and even contain PCI and non-PCI feature gated types internally.

kernel/src/drivers/fs/mod.rs

Lines 1 to 2 in 70ad8ef

	#[cfg(feature = "pci")]
	pub mod virtio_fs;

kernel/src/drivers/fs/virtio_fs.rs

Lines 15 to 18 in 70ad8ef

	#[cfg(not(feature = "pci"))]
	use crate::drivers::virtio::transport::mmio::{ComCfg, IsrStatus, NotifCfg};
	#[cfg(feature = "pci")]
	use crate::drivers::virtio::transport::pci::{ComCfg, IsrStatus, NotifCfg};

[hcsch]

I might have been a bit too much in an issue opening spree, sorry. It seems this was intentionally done in b5a45e2 to hide VIRTIO fs when the PCI feature has not been selected.

[mkroening]

I think that was done to please clippy. Integrating and enabling virtio-fs on MMIO would be good. 👍

[elise-insall]

Hello, I'm a student taking virtualization at UT Austin. Our class requires us to contribute to an open source project. Can my group and I take this one?

[mkroening]

Sure, go ahead!

If you have any questions, feel free to ask them here or chat with us on our Zulip. :)