gettext: fix ngettext usage

_() should not be used in conjunction with ngettext(),
otherwise it's impossible to properly translate the message into
languages that have multiple plural forms, such as Czech.

Additionally, spice-gtk must use dngettext instead of ngettext,
otherwise the translation strings are searched within a wrong domain.

Fix ngettext usage in usb-device-widget.c and
update the PO files accordingly.

Signed-off-by: Jakub Janků <jjanku@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>