qapi: convert sendkey

Convert 'sendkey' to use QAPI.

QAPI passes key's index of mapping table to qmp_send_key(),
not keycode. So we use help functions to convert key/code to
index of key_defs, and 'index' will be converted to 'keycode'
inside qmp_send_key().

For qmp, QAPI would check invalid key and raise error.
For hmp, invalid key is checked in hmp_send_key().

'send-key' of QMP doesn't support key in hexadecimal format.

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