vmm: Add a "discard_writes=" to --pmem

This opens the backing file read-only, makes the pages in the mmap()
read-only and also makes the KVM mapping read-only. The file is also
mapped with MAP_PRIVATE to make the changes local to this process only.

This is functional alternative to having support for making a
virtio-pmem device readonly. Unfortunately there is no concept of
readonly virtio-pmem (or any type of NVDIMM/PMEM) in the Linux kernel so
to be able to have a block device that is appears readonly in the guest
requires significant specification and kernel changes.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

src/main.rs

@@ -173,7 +173,7 @@ fn create_app<'a, 'b>(
                 .help(
                     "Persistent memory parameters \
                      \"file=<backing_file_path>,size=<persistent_memory_size>,iommu=on|off,\
-                     mergeable=on|off\"",
+                     mergeable=on|off,discard_writes=on|off,\"",
                 )
                 .takes_value(true)
                 .min_values(1)