[vmware] Fix length returned by guestrpc_command()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/vmware/guestrpc.c

 			if ( status_len ) {
 				*(status_bytes++) = ( data & 0xff );
 				status_len--;
+				len--;
 			} else if ( reply_len ) {
 				*(reply_bytes++) = ( data & 0xff );
 				reply_len--;