X-Men: Days Of Future Past writer Simon Kinberg comments on the perceived Quicksilver controversy as both Fox Studios and Marvel Studios are using the character in their films.
(As Quicksilver is considered both a part of the X-Men universe and Avengers universe the character can be used by both studios)
Kinberg mentions in the following video that Fox already had Quicksilver in their script before knowing that he was in The Avengers 2. He also mentions they cut out a Scarlet Witch reference from X-Men: Days Of Future Past, and that "[Quicksilver] will absolutely be back for the next X-Men movie."
Of course, the next X-Men movie is X-Men: Apocalypse which has a May 27, 2016 release.
Quicksilver is played by Evan Peters.