virt: Fix remote login matching Last Login

Paolo Bonzini noticed our remote handle_prompts code matching the
"Last Login: blah" line which is common to many distros.  This
fixes the problem by using a "negative lookbehind", i.e. it
matches "Login:" only if it is NOT preceeded by a match of
"Last:".

Signed-off-by: Chris Evich <cevich@redhat.com>