virtio: remove virtio_alloc_virtqueues' flags arg
Summary:
the flags argument is unused.
Its initial design idea has been superceded by the addition of
virtio_setup_intr and related APIs.
Sponsored by: The FreeBSD Foundation
Reviewers: bryanv
Reviewed By: bryanv
Subscribers: cognet, imp
Differential Revision: https://reviews.freebsd.org/D41850