If you aren't a citizen, then you can't vote.  Voting is a right for citizens only.  So, absolutely, the form should ask whether or not you are a citizen.

Further, Virginia law requires that it be asked:

"The form of the application to register shall require the applicant to provide the following information: full name; gender; date of birth; social security number, if any; whether the applicant is presently a United States citizen; address of residence in the precinct; place of last previous registration to vote; and whether the applicant has ever been adjudicated incapacitated or convicted of a felony, and if so, under what circumstances the applicant’s right to vote has been restored."  § 24.2-418. Application for registration.