virtio.h: add some user-friendly apis

[CV-Bowen]

• virtio.h: add new api virtio_has_feature()

virtio_has_feature() can be easily used to heck if the virtio device
support a specific feature.

And assgin feature to vdev->feature for virtio device role when get
features, so the virtio device side can use virtio_has_featrue() to
check weather the virtio device support a feature.

• virtio.h: add new feature bit VIRTIO_F_ANY_LAYOUT

Follow the virtio spec, this feature bit indicates that the device
accepts arbitrary descriptor layouts.

[CV-Bowen]

@arnopo Hi, I meet a CI issue:

#define virtio_read_config_member(vdev, structname, member, _ptr) \
	virtio_read_config((vdev), metal_offset_of(structname, member), \
			   _ptr, sizeof(*(_ptr)))

This code reports MACRO_ARG_REUSE: Macro argument reuse '_ptr' - possible side-effects?
So can we use typeof to avoid this issue?

[arnopo]

This function return valid value only if virtio_negotiate_features has been called first . That also means that it does not work for virtio device role.

[CV-Bowen]

Yes, I consider assign back the final features to vdev->features when call virtio_get_feature() for VIRTIO_DEVICE side, so VIRTIO_DEVICE can use this virtio_has_feature() after called virtio_get_feature(). What do you think?

[CV-Bowen]

Done, @arnopo Could you review again?

[arnopo]

Yes, I consider assign back the final features to vdev->features when call virtio_get_feature() for VIRTIO_DEVICE side, so VIRTIO_DEVICE can use this virtio_has_feature() after called virtio_get_feature(). What do you think?

This does not prevent the use case where virtio_has_feature is called before virtio_get_features.
to be safe virtio_has_feature should call virtio_get_features if vdev->features == NULL

[CV-Bowen]

@glneo @arnopo Please review again.

[arnopo]

Yes, I consider assign back the final features to vdev->features when call virtio_get_feature() for VIRTIO_DEVICE side, so VIRTIO_DEVICE can use this virtio_has_feature() after called virtio_get_feature(). What do you think?

This does not prevent the use case where virtio_has_feature is called before virtio_get_features.
to be safe virtio_has_feature should call virtio_get_features if vdev->features == NULL