configure: Fix bad shell expression for non-Linux hosts

With vhost_net="" (most non-Linux hosts), configure prints an
error message:

test: 2551: =: unexpected operator

Fix this and similar code by adding the missing "".

Cc: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
