add support for hypercalls

xencall/examples/simple.rs

@@ -0,0 +1,11 @@
+use std::ffi::c_ulong;
+use std::ptr::addr_of;
+use xencall::{XenCall, XenCallError};
+
+fn main() -> Result<(), XenCallError> {
+    let mut call = XenCall::open()?;
+    let message = "Hello World";
+    let bytes = message.as_bytes();
+    call.hypercall3(18, 0, bytes.len() as c_ulong, addr_of!(bytes) as c_ulong)?;
+    Ok(())
+}