Before the qapi conversion, the sendkey command could be used to
send key codes in hex directly to the guest. In HMP, this would
be like:

 (qemu) sendkey 0xdc

However, the qapi conversion broke this, as it only supports sending
QKeyCode values to the guest. That's a regression.

This commit fixes the problem by adding hex value support down
the QMP interface, qmp_send_key().

In more detail, this commit:

 1. Adds the KeyValue union. This can represent an hex value or
    a QKeyCode value

 2. *Changes* the QMP send-key command to take an KeyValue argument
    instead of a QKeyCode one

 3. Adapt hmp_send_key() to the QMP interface changes

Item 2 is an incompatible change, but as we're in development phase
(and this command has been merged a few weeks ago) this shouldn't be
a problem.

Finally, it's not possible to split this commit without breaking the
build.

Reported-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>