Not always a prerequisite but generally a committer becomes a PMC member (see on how to become a committer: DRAFT).
Other references about PMC membership and CloudStack project bylaws and other links and docs:
Becoming a PMC member, it largely a support building exercise where other PMC members can review anybody in the community, discuss/propose, vote and invite them to become a PMC member.
Some criteria could be following:
Bottomline: it's really upto the PMC members to maintain a healthy and active PMCs and committers base in the project by regularly reviewing, proposing, voting and inviting new PMC members and committers. Any PMC member can discuss and propose a candidate on the private@ mailing list, review with other PMC members and build support and vote.
Misc links and references: