guestfs: Don't report exception if there's read access to kernel

Commit 92ae0f1 ("libvirt - Add log if libguestfs can't read host
kernel") reworks the logic of handling access to Kernel for libguestfs.
In doing that, it erroneously raises an exception when libguestfs is
_able_ to access the Kernel.

Fix it by reporting exception only when libguestfs does _not_ have
read access to the Kernel.

This was first tried by Kevin Zhao here
Ic6802650cb8f93e0d02c51e9014eb85a7e71f6fe, but is abandoned for some
reason.

This also adds a little more direction on what to do to fix this error.

Related-Bug: #1646002

Change-Id: Id6b4108e4e4af7c98b3e1bd9de3a09ad057e7b92

nova/tests/unit/virt/disk/vfs/test_guestfs.py

@@ -320,6 +320,7 @@ class VirtDiskVFSGuestFSTest(test.NoDBTestCase):
         m = mock.MagicMock()
         m.launch.side_effect = Exception
         vfs = vfsimpl.VFSGuestFS(self.qcowfile)
+        mock_access.return_value = False
         with mock.patch('eventlet.tpool.Proxy', return_value=m):
             self.assertRaises(exception.LibguestfsCannotReadKernel,
                                     vfs.inspect_capabilities)

nova/virt/disk/vfs/guestfs.py

@@ -80,8 +80,11 @@ class VFSGuestFS(vfs.VFS):
             g.add_drive("/dev/null")  # sic
             g.launch()
         except Exception as e:
-            if os.access("/boot/vmlinuz-%s" % os.uname()[2], os.R_OK):
-                raise exception.LibguestfsCannotReadKernel()
+            kernel_file = "/boot/vmlinuz-%s" % os.uname()[2]
+            if not os.access(kernel_file, os.R_OK):
+                raise exception.LibguestfsCannotReadKernel(
+                    _("Please change permissions on %s to 0x644")
+                    % kernel_file)
             raise exception.NovaException(
                 _("libguestfs installed but not usable (%s)") % e)