qemu: build w/o virtfs support

[thomas]

To use the virtfs support directly qemu needs to be run as root, what we don't do here.
The related virtfs-proxy-helper(1) would require root privileges at launch time, also not part of our scenarios.

[donald]

Sure? Sound like you can use that without root, only certain operations like chown need privileges on the host, as could be expected.

http://manpages.ubuntu.com/manpages/trusty/man1/virtfs-proxy-helper.1.html

"Pass-through security model in QEMU 9p server needs root privilege to do few file operations (like chown, chmod to any mode/uid:gid)."