Monitor: Introduce the qmp-commands.hx file

Monitor: Introduce the qmp-commands.hx file

This file contains a copy of the following information from the
qemu-monitor.hx file:

    o QObject handlers entries
    o QMP documentation (all SQMP/EQMP sections)

Right now it's only used to generate the QMP docs in QMP/, but
next commits will turn this into QMP's command dispatch table.

It's important to note that QObject handlers entries are going
to get duplicated: they will exist in both QMP's and HMP's
dispatch tables.

This will be fixed in the near future, when we add a proper
QMP call interface and HMP is converted to use it. This way we
can completely drop QObject handlers entries from HMP's tables.

NOTE: HMP specific constructions, like "q|quit", have been dropped.

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