New API: guestfs_device_name returning the drive name

For each drive added, return the name.  For example calling this with
index 0 will return the string "/dev/sda".  I called it
guestfs_device_name (not drive_name) for consistency with the existing
guestfs_device_index function.

You don't really need to call this function.  You can follow the
advice here:
https://libguestfs.org/guestfs.3.html#block-device-naming
and assume that drives are added with predictable names like
"/dev/sda", "/dev/sdb", etc.

However it's useful to expose the internal guestfs_int_drive_name
function since especially handling names beyond index 26 is tricky
(https://rwmj.wordpress.com/2011/01/09/how-are-linux-drives-named-beyond-drive-26-devsdz/)

Fixes: https://github.com/libguestfs/libguestfs/issues/80
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

lib/drives.c

@@ -31,6 +31,7 @@
 #include <netdb.h>
 #include <arpa/inet.h>
 #include <assert.h>
+#include <errno.h>
 #include <libintl.h>
 
 #include "c-ctype.h"
@@ -1084,3 +1085,17 @@ guestfs_impl_device_index (guestfs_h *g, const char *device)
     error (g, _("%s: device not found"), device);
   return r;
 }
+
+char *
+guestfs_impl_device_name (guestfs_h *g, int index)
+{
+  char drive_name[64];
+
+  if (index < 0 || index >= g->nr_drives) {
+    guestfs_int_error_errno (g, EINVAL, _("drive index out of range"));
+    return NULL;
+  }
+
+  guestfs_int_drive_name (index, drive_name);
+  return safe_asprintf (g, "/dev/sd%s", drive_name);
+}