ipmi: remove IPMI_CHECK_CMD_LEN() macro

Most IPMI command handlers in the BMC simulator start with a call to
the macro IPMI_CHECK_CMD_LEN() which verifies that a minimal number of
arguments expected by the command are indeed available. To achieve
this task, the macro implicitly uses local variables which is
misleading in the code.

This patch adds a 'cmd_len_min' attribute to the struct IPMICmdHandler
defining the minimal number of arguments expected by the command and
moves this check in the global command handler ipmi_sim_handle_command().

To clarify the checks being done on the received command, the patch
introduces a helper ipmi_get_handler().

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
