ppc: Clean up error handling in ppc_set_compat()

ppc: Clean up error handling in ppc_set_compat()

Current ppc_set_compat() returns -1 for errors, and also (unconditionally)
reports an error message.  The caller in h_client_architecture_support()
may then report it again using an outdated fprintf().

Clean this up by using the modern error reporting mechanisms.  Also add
strerror(errno) to the error message.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>