configure: Assure printing "yes" or "no" for VirtFS support

When auto-detecting VirtFS support, virtfs="". Set it to "no" after
checking whether it was explicitly requested through --enable-virtfs.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
