Update the MILC HISQ MG interface to support initial guesses

[weinbe2]

The current interface doesn't support initial guesses because of a sign convention difference between MILC and QUDA. The fix is flipping the sign on an initial guess before handing it in (this is what's done on the result); since MG is generally quick enough on its own I left out the overhead of the sign flip.

This is nonetheless easy enough to fix; I want to fix it more performantly by flipping the sign on the device as opposed to the host, which is just some plumbing work. This issue is a reminder-to-self.