virkeycode: Allow ANSI_A

virkeycode: Allow ANSI_A

https://bugzilla.redhat.com/show_bug.cgi?id=1044806

Currently, sending the ANSI_A keycode from os_x codepage doesn't work as
it has a special value of 0x0. Our internal code handles that no
different to other not defined keycodes. Hence, in order to allow it we
must change all the undefined keycodes from 0 to -1 and adapt some code
too.

  # virsh send-key guestname --codeset os_x ANSI_A
  error: invalid keycode: 'ANSI_A'

  # virsh send-key guestname --codeset os_x ANSI_B
  # virsh send-key guestname --codeset os_x ANSI_C

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>