Even if Virginia had emancipated their slaves long before the ACW it wouldn't mean that they would fight for the Union. Virginia's secession didn't come about due to the Slavery being threatened, it came about due to the Lincoln demanding that Virginia call men up into the Union armies to invade and forcably bring back into the Union the already seceeded states. The same is true of Arkansas, Tennessee and North Carolina, all of those states only seceeded when Lincoln call for troops.
If Virginia couldn't bring themselves to fight for the slaves states in the 1860's they would likely still refuse to fight for the Union and be neutral, creating a bit of a problem for the Union forces if Kentucky is neutral as well.
Thats assuming everything else is the same as well. Which it won't be. With Virginia abolishing slavery, at least some other states will likely follow. And the various debates about slavery in congress will go differently, leading to different laws, leading to a completely different political system by the time 1860 rolls around.