If a group was being looked by using sysdb_search_by_name() in a MPG
domain, the code would search only for group objects -- but in a MPG
domain, there may be none, the groups are typically inferred from a user
object.

This could have caused issues e.g. for IPA code with the following
sequence:

getent group aduser@windows.domain
getent passwd aduser@windows.domain

The former would fail to add the fallback subdomain homedir and the latter
would then return a user entry without a homedir, with libc falling back to
the "/" homedir.

NOTE: The patch has no unit test. I'll add one when the approach is
confirmed as correct by another developer.