qmp: Clean up how we enforce capability negotiation

To enforce capability negotiation before normal operation,
handle_qmp_command() inspects every command before it's handed off to
qmp_dispatch().  This is a bit of a layering violation, and results in
duplicated code.

Before capability negotiation (!cur_mon->in_command_mode), we fail
commands other than "qmp_capabilities".  This is what enforces
capability negotiation.

Afterwards, we fail command "qmp_capabilities".

Clean this up as follows.

The obvious place to fail a command is the command itself, so move the
"afterwards" check to qmp_qmp_capabilities().

We do the "before" check in every other command, but that would be
bothersome.  Instead, start with an alternate list of commands that
contains only "qmp_capabilities".  Switch to the full list in
qmp_qmp_capabilities().

Additionally, replace the generic human-readable error message for
CommandNotFound by one that reminds the user to run qmp_capabilities.
Without that, we'd regress commit 2d5a8346.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1488544368-30622-8-git-send-email-armbru@redhat.com>
[Mirco-optimization squashed in, commit message typo fixed]
Reviewed-by: Eric Blake <eblake@redhat.com>