selftests/bpf: vm: Add support for VIRTIO_FS

danobi/vmtest is going to migrate from using 9p to using virtio_fs to
mount the local rootfs: https://github.com/danobi/vmtest/pull/88



BPF CI uses danobi/vmtest to run bpf selftests and will need to support
VIRTIO_FS.

This change enables new kconfigs to be able to support the upcoming
danobi/vmtest.

Tested by building a new kernel with those config and confirming it
would successfully run with 9p (currently what is used by vmtest), and
with virtio_fs (using a local build of vmtest).

  $ vmtest -k arch/x86/boot/bzImage "findmnt /"
  => bzImage
  ===> Booting
  ===> Setting up VM
  ===> Running command
  TARGET SOURCE    FSTYPE OPTIONS
  /      /dev/root 9p     rw,relatime,cache=5,access=client,msize=512000,trans=virtio
  $ /home/chantra/local/danobi-vmtest/target/debug/vmtest -k arch/x86/boot/bzImage "findmnt /"
  => bzImage
  ===> Initializing host environment
  ===> Booting
  ===> Setting up VM
  ===> Running command
  TARGET SOURCE FSTYPE   OPTIONS
  /      rootfs virtiofs rw,relatime

Changes in v2:
* Sorted configs alphabetically

Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/20240925002210.501266-1-chantr4@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>