vfio/pci: Use local error object in vfio_initfn

vfio/pci: Use local error object in vfio_initfn

To prepare for migration to realize, let's use a local error
object in vfio_initfn. Also let's use the same error prefix for all
error messages.

On top of the 1-1 conversion, we start using a common error prefix for
all error messages. We also introduce a similar warning prefix which will
be used later on.

Signed-off-by: Eric Auger <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Alex Williamson <[email protected]>