docs/system: convert managed startup to rST.

Fix one typo in the process and format more option and
command names as literal text, but make no significant
changes to the content.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-18-peter.maydell@linaro.org
Message-id: 20200226113034.6741-17-pbonzini@redhat.com

docs/system/index.rst

@@ -13,6 +13,6 @@ Contents:
 
 .. toctree::
    :maxdepth: 2
-
+   managed-startup
    security
    vfio-ap

docs/system/managed-startup.rst

@@ -0,0 +1,35 @@
+Managed start up options
+========================
+
+In system mode emulation, it's possible to create a VM in a paused
+state using the ``-S`` command line option. In this state the machine
+is completely initialized according to command line options and ready
+to execute VM code but VCPU threads are not executing any code. The VM
+state in this paused state depends on the way QEMU was started. It
+could be in:
+
+- initial state (after reset/power on state)
+- with direct kernel loading, the initial state could be amended to execute
+  code loaded by QEMU in the VM's RAM and with incoming migration
+- with incoming migration, initial state will be amended with the migrated
+  machine state after migration completes
+
+This paused state is typically used by users to query machine state and/or
+additionally configure the machine (by hotplugging devices) in runtime before
+allowing VM code to run.
+
+However, at the ``-S`` pause point, it's impossible to configure options
+that affect initial VM creation (like: ``-smp``/``-m``/``-numa`` ...) or
+cold plug devices. The experimental ``--preconfig`` command line option
+allows pausing QEMU before the initial VM creation, in a "preconfig" state,
+where additional queries and configuration can be performed via QMP
+before moving on to the resulting configuration startup. In the
+preconfig state, QEMU only allows a limited set of commands over the
+QMP monitor, where the commands do not depend on an initialized
+machine, including but not limited to:
+
+- ``qmp_capabilities``
+- ``query-qmp-schema``
+- ``query-commands``
+- ``query-status``
+- ``x-exit-preconfig``