qemu-options: document existance of versioned machine types

The -machine docs did not explain what the versioned machine
types are for, nor that they'll be maintained across
releases.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170725141041.1195-1-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-options.hx

@@ -49,7 +49,20 @@ STEXI
 @item -machine [type=]@var{name}[,prop=@var{value}[,...]]
 @findex -machine
 Select the emulated machine by @var{name}. Use @code{-machine help} to list
-available machines. Supported machine properties are:
+available machines.
+
+For architectures which aim to support live migration compatibility
+across releases, each release will introduce a new versioned machine
+type. For example, the 2.8.0 release introduced machine types
+``pc-i440fx-2.8'' and ``pc-q35-2.8'' for the x86_64/i686 architectures.
+
+To allow live migration of guests from QEMU version 2.8.0, to QEMU
+version 2.9.0, the 2.9.0 version must support the ``pc-i440fx-2.8''
+and ``pc-q35-2.8'' machines too. To allow users live migrating VMs
+to skip multiple intermediate releases when upgrading, new releases
+of QEMU will support machine types from many previous versions.
+
+Supported machine properties are:
 @table @option
 @item accel=@var{accels1}[:@var{accels2}[:...]]
 This is used to enable an accelerator. Depending on the target architecture,