volatile_memory: Deprecate .as_ptr() interfaces

The `.as_ptr()` interface can't be used with the `xen` feature. Mark
them deprecated with a note to use the new interfaces.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Author: vireshk

CHANGELOG.md

@@ -15,6 +15,14 @@
 - [[#241]](https://github.com/rust-vmm/vm-memory/pull/245) mmap_xen: Don't drop
   the FileOffset while in use #245
 
+## [Unreleased]
+
+### Deprecated
+
+- [[#244]](https://github.com/rust-vmm/vm-memory/pull/241) Deprecate volatile
+  memory's `as_ptr()` interfaces. The new interfaces to be used instead are:
+  `ptr_guard()` and `ptr_guard_mut()`.
+
 ## [v0.12.0]
 
 ### Added

src/volatile_memory.rs

@@ -355,6 +355,10 @@ impl<'a, B: BitmapSlice> VolatileSlice<'a, B> {
     /// Returns a pointer to the beginning of the slice. Mutable accesses performed
     /// using the resulting pointer are not automatically accounted for by the dirty bitmap
     /// tracking functionality.
+    #[deprecated(
+        since = "0.12.1",
+        note = "Use `.ptr_guard()` or `.ptr_guard_mut()` instead"
+    )]
     #[cfg(not(all(feature = "xen", unix)))]
     pub fn as_ptr(&self) -> *mut u8 {
         self.addr
@@ -1028,6 +1032,10 @@ where
     /// Returns a pointer to the underlying memory. Mutable accesses performed
     /// using the resulting pointer are not automatically accounted for by the dirty bitmap
     /// tracking functionality.
+    #[deprecated(
+        since = "0.12.1",
+        note = "Use `.ptr_guard()` or `.ptr_guard_mut()` instead"
+    )]
     #[cfg(not(all(feature = "xen", unix)))]
     pub fn as_ptr(&self) -> *mut u8 {
         self.addr as *mut u8
@@ -1217,6 +1225,10 @@ where
     /// Returns a pointer to the underlying memory. Mutable accesses performed
     /// using the resulting pointer are not automatically accounted for by the dirty bitmap
     /// tracking functionality.
+    #[deprecated(
+        since = "0.12.1",
+        note = "Use `.ptr_guard()` or `.ptr_guard_mut()` instead"
+    )]
     #[cfg(not(all(feature = "xen", unix)))]
     pub fn as_ptr(&self) -> *mut u8 {
         self.addr