The old Virginia Senate districts were drawn by Democrats in 2011 in an effort to try and maintain their then four seat majority. Republicans were up in arms and for good reason. Republicans were only able to win 20 out of 40 seats in 2011, despite the fact that they won the statewide vote for senate 57% to 40% in the 2011 elections. That result is far more skewed than it was for the national house in 2012. If percentage of vote equaled percentage of seats, Virginia Republicans should have won 23 to 24 seats in 2011. Usually, though, in single member districts, the curve is more responsive and 57% of the vote would result in something like 60-65% or 24 to 26 of the 40 seats.
The new districts are more likely to mirror the statewide vote for state senate. One estimate from Ben Tribbett has Republicans winning up to 27 seats in a good Republican year. This isn't terribly far off from the upper range of the responsive curve for the 2011 elections, and it certainly is closer to statewide vote than the actual 2011 seat makeup. My friend Sean Trende puts the estimate at locking in a Republican majority by two to three seats. This would actually be at the lower end of the proportional curve, but still closer to what we'd expect than the actual 2011 seat makeup.