Add KRUN_FS_ROOT_TAG define and document krun_set_root alternative

Expose the virtiofs tag we use internally to krun_set_root() so that
users are able to use it with APIs such as krun_add_virtiofs3()
(recently added via #623). This way, they can gain control over the
read-only state of the root file system (among other properties) without
having to guess the tag to use.

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

include/libkrun.h

@@ -97,9 +97,18 @@ int32_t krun_free_ctx(uint32_t ctx_id);
  */
 int32_t krun_set_vm_config(uint32_t ctx_id, uint8_t num_vcpus, uint32_t ram_mib);
 
+/**
+ * The virtiofs tag used for the root filesystem. Can be used with krun_add_virtiofs*
+ * for more control over root filesystem parameters (e.g. read-only, DAX window size).
+ */
+#define KRUN_FS_ROOT_TAG "/dev/root"
+
 /**
  * Sets the path to be use as root for the microVM. Not available in libkrun-SEV.
  *
+ * For more control over the root filesystem (e.g. read-only, DAX window size),
+ * use krun_add_virtiofs3() with KRUN_FS_ROOT_TAG instead.
+ *
  * Arguments:
  *  "ctx_id"    - the configuration context ID.
  *  "root_path" - a null-terminated string representing the path to be used as root.