qemu-ga: Add interface to traverse the qmp command list by QmpCommand

In the original code, qmp_get_command_list is used to construct
a list of all commands' name. To get the information of all qga
commands, it traverses the name list and search the command info
with its name.  So it can cause O(n^2) in the number of commands.

This patch adds an interface to traverse the qmp command list by
QmpCommand to replace qmp_get_command_list. It can decrease the
complexity from O(n^2) to O(n).

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
*fix up commit subject
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>