configure: Reindent VirtFS check

Avoid tab-indention and fit in with the surrounding code.

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>
