[Virtio-fs] support booting of arbitrary non-blockdevice file systems v2

[Christoph Hellwig]

Hi all,

this series adds support to boot off arbitrary non-blockdevice root file
systems, based off an earlier patch from Vivek.

Chances since v1:
 - don't try to mount every registered file system if none is specified
 - fix various null pointer dereferences when certain kernel paramters are
   not set
 - general refactoring.