qga: distinguish binary modes in "guest_file_open_modes" map

qga: distinguish binary modes in "guest_file_open_modes" map

In Windows guests this may make a difference.

Since the original patch (commit c689b4f1) sought to be pedantic and to
consider theoretical corner cases of portability, we should fix it up
where it failed to come through in that pursuit.

Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>