kvm tools: Use kvm__get_dir() helper function

This patch adds a kvm__get_dir() helper function that replaces open-coded uses
of HOME_DIR and KVM_PID_FILE_PATH.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>