chore(xen): split platform support into separate crate (#195)

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "crates/xen/xenclient",
     "crates/xen/xenevtchn",
     "crates/xen/xengnt",
+    "crates/xen/xenplatform",
     "crates/xen/xenstore",
 ]
 resolver = "2"
@@ -32,6 +33,7 @@ backhand = "0.15.0"
 base64 = "0.22.1"
 byteorder = "1"
 bytes = "1.5.0"
+c2rust-bitfields = "0.18.0"
 cgroups-rs = "0.3.4"
 circular-buffer = "0.1.7"
 comfy-table = "7.1.1"