monorepo: Unify .gitignore file

Join the entries of `.gitignore` from `vfio` and `vfio-user` as the
final `.gitignore` of `vfio` monorepo.

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

.gitignore

@@ -1,4 +1,6 @@
+**/*.rs.bk
 .idea
 .vscode
-target
+/target
 Cargo.lock
+kcov_output