pci_add storage: fix error handling for 'if' parameter (Eduardo Habkost)

pci_add storage: fix error handling for 'if' parameter (Eduardo Habkost)

This fixes:

 - The error message to show the actual if= argument value. It was showing
   the filename instead, because 'buf' is reaused on the filename parsing.
 - A bug that makes a block device to be created even when an unsupported if= arg
   is passed to pci_add.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6981 c046a42c-6fe2-441c-8c8c-71466251a162