This isn't a virus!
This a registry entry, but some values in the registry MBAM considers bad, this could have been changes a user made or in this case possible corruption, see the difference between the two.
Bad (%fystemroot%\system32\svchost.exe -k netsvcs)
Good: (%SystemRoot%\System32\svchost.exe -k netsvcs)
Can you see it %f and not %S.
The %SystemRoot% is a variable for C:\Windows now the %fystemroot% isn't a valid variable so it would have no assigned value, so is effectively dead in the water and can't do anything.
How this became corrupt I haven't the slightest idea, but avast doesn't look for registry problems in this way, avast scans your system and if it finds infected files then it would look for associated registry entries for those files.
# MBAM doesn't actually delete the registry entry as far as I'm aware it should chnage it but keep a copy of the original registry key in the quarantine area of MBAM.
If you had regedit open whilst making the decision about this and MBAM said quaranteened and deleted the item may still appear, close the regedit and open it again. If it is still the same you could manually change the value.
From:
%fystemroot%\system32\svchost.exe -k netsvcs
To:
%SystemRoot%\system32\svchost.exe -k netsvcs