I would write “Log in to host.com.”
I think that “login” is a noun (as in “login screen”). I would find the words “loginned” and “loginning” awkward.
As for “Log in to host.com” versus “Log into host.com,” I would use the former because I think that “log in” is a fixed phrase. Martha’s answer to another question is also related.
Added: The Corpus of Contemporary American English (COCA) lists 65 occurrences of “log in to” and 58 occurrences of “log into,” both including inflected forms and excluding the Spoken section. (The queries used are
[log].[v*] in to and
[log].[v*] into.) Since “log in to” is also used in context like “log in to download it,” the actual number of occurrences of “log in to [host]” is slightly smaller than 65. In any case, it suggests that the phrase “log into” is also used commonly, although I am not sure how good it is to use COCA to compare technical terms.