QMP: Introduce commands documentation

One of the most important missing feature in QMP today is its
supported commands documentation.

The plan is to make it part of self-description support, however
self-description is a big task we have been postponing for a
long time now and still don't know when it's going to be done.

In order not to compromise QMP adoption and make users' life easier,
this commit adds a simple text documentation which fully describes
all QMP supported commands.

This is not ideal for a number of reasons (harder to maintain,
text-only, etc) but does improve the current situation. To avoid at
least divering from the user monitor help and texi snippets, QMP bits
are also maintained inside qemu-monitor.hx, and hxtool is extended to
generate a single text file from them.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
