block/ssh: Add ssh_has_filename_options_conflict()

We have 5 options plus ("server") option which is added in the next
patch that conflict with specifying a SSH filename. We need to iterate
over all the options to check whether its key has an "server." prefix.

This iteration will help us adding the new option "server" easily.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
