qga: update docs with systemd suspend support info

Commit 067927d6 ("qga: systemd hibernate/suspend/hybrid-sleep
support") failed to update qapi-schema.json after adding systemd
hibernate/suspend/hybrid-sleep capabilities to guest-suspend-* QGA
commands.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>