Who are the "important" people in Kabul? Why are they called "guests"?
Farid calls Amir a "guest" in Kabul because he is a Pashtun; a member of the privileged, or "important", class in Afghanistan. Pashtuns represent 42% of the population and dominate governmental and legislative bodies. While there isn't a rigid caste system in Afghanistan, the diversity of ethnic cultures does create distinct socio-economical situations.
As we see in The Kite Runner, Hazaras are viewed as being at the bottom of the ethnic hierarchy. Hazaras typically hold service level jobs and make a menial living.
Amir is a guest in Kabul because as Baba's son, a well-respected and successful Pashtun, he has lived a very sheltered and protected life compared to other citizens of different ethnic groups. Amir grew up in a big house guarded by gates which blocked out the "real" experience of Kabul.
Amnesty international has a great guide for The Kite Runner with a very comprehensive breakdown of the ethnic hierarchy in Afghanistan. I sincerely suggest you check it out.
The flashbacks used by author Khaled Hosseini in The Kite Runner serve to remind the reader of the good times Amir had before the Russian invasion. He visits his old house and surrounding property to relive his youthful experiences (just as many people do today when they revisit their old homes after a long absence) as a boy and with Hassan. Amir is remembering how life was before the invasion of the Russians and, later, the Taliban. He is hopeful of making things right again by locating Sohrab and bringing him back home.