hw/ppc/pnv: Remove superfluous "qemu" prefix from error strings

error_report() already puts a prefix with the program name in front
of the error strings, so the "qemu:" prefix is not necessary here
anymore.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>