docs/interop/live-block-operations: Do not hard-code the QEMU binary …

…name

In downstream, we want to use a different name for the QEMU binary,
and some people might also use the docs for non-x86 binaries, that's
why we already created the |qemu_system| placeholder in the past.
Use it now in the live-block-operations doc, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210607172311.915385-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/interop/live-block-operations.rst

@@ -127,13 +127,15 @@ Interacting with a QEMU instance
 
 To show some example invocations of command-line, we will use the
 following invocation of QEMU, with a QMP server running over UNIX
-socket::
+socket:
 
-    $ ./qemu-system-x86_64 -display none -no-user-config \
-        -M q35 -nodefaults -m 512 \
-        -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \
-        -device virtio-blk,drive=node-A,id=virtio0 \
-        -monitor stdio -qmp unix:/tmp/qmp-sock,server=on,wait=off
+.. parsed-literal::
+
+  $ |qemu_system| -display none -no-user-config -nodefaults \\
+    -m 512 -blockdev \\
+    node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \\
+    -device virtio-blk,drive=node-A,id=virtio0 \\
+    -monitor stdio -qmp unix:/tmp/qmp-sock,server=on,wait=off
 
 The ``-blockdev`` command-line option, used above, is available from
 QEMU 2.9 onwards.  In the above invocation, notice the ``node-name``
@@ -692,14 +694,16 @@ And start the destination QEMU (we already have the source QEMU running
 -- discussed in the section: `Interacting with a QEMU instance`_)
 instance, with the following invocation.  (As noted earlier, for
 simplicity's sake, the destination QEMU is started on the same host, but
-it could be located elsewhere)::
-
-    $ ./qemu-system-x86_64 -display none -no-user-config \
-        -M q35 -nodefaults -m 512 \
-        -blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \
-        -device virtio-blk,drive=node-TargetDisk,id=virtio0 \
-        -S -monitor stdio -qmp unix:./qmp-sock2,server=on,wait=off \
-        -incoming tcp:localhost:6666
+it could be located elsewhere):
+
+.. parsed-literal::
+
+  $ |qemu_system| -display none -no-user-config -nodefaults \\
+    -m 512 -blockdev \\
+    node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \\
+    -device virtio-blk,drive=node-TargetDisk,id=virtio0 \\
+    -S -monitor stdio -qmp unix:./qmp-sock2,server=on,wait=off \\
+    -incoming tcp:localhost:6666
 
 Given the disk image chain on source QEMU::