add -drive if=none

This adds a host drive, but doesn't implicitly add a guest drive for it.
First step in splitting host and guest configuration, check the
following patches to see how this can be used ...

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
