monitor: move key_defs[] table and introduce two help functions

This patch added two help functions to convert key/code to index of
mapping table, those functions will return Q_KEY_CODE_MAX if the
code/key is invalid.

Patch also moved key_defs[] to input.c, and removed useless KeyDef struct.
Key's index in QKeyCode enmu is same as keycode's index in new key_defs[].
Monitor functions were changed to access key_defs[] directly.

key_defs[] is used in do_send_key(), so export key_defs[]. It will be
changed to static in next patch.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
