pci-assign: Fix error_report of pci-stub message

pci-assign: Fix error_report of pci-stub message

Using multiple calls to error_report here means every line is
prefaced with the (potentially long) pci-assign command line
arguments.

Use a single error_printf to preserve the intended formatting.
Since this code path is always preceded by an error_report call,
we don't lose the command line reporting.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>