This how the law of Bahrain looks like, I am confident that the Qatari law would not be much different:

"the foreign worker has the right- without requiring the consent of the employer- to move from one employer to another without breaching the rights of the employer as per the articles of the law or the clauses of the employment contract between the two sides"

So, go and dig that contract out, and read what it says. Employers will now make sure that the new contracts are tightly written to protect their interest.

Complete Bahraini law is here:
http://portal.lmra.bh/english/legal/show/17