add support for hypercalls

xencall/src/sys.rs

@@ -0,0 +1,11 @@
+use nix::{ioctl_readwrite_bad, request_code_none};
+use std::ffi::{c_long, c_ulong};
+
+#[repr(C)]
+pub struct Hypercall {
+    pub op: c_ulong,
+    pub arg: [c_ulong; 5],
+    pub retval: c_long,
+}
+
+ioctl_readwrite_bad!(hypercall, request_code_none!(b'E', 0), Hypercall);