iotests: Include QMP input in .out files

iotests: Include QMP input in .out files

We generally include relevant HMP input in .out files, by virtue of
the fact that HMP echoes its input.  But QMP does not, so we have to
explicitly inject it in the output stream (appropriately filtered to
keep the tests passing), in order to make it easier to read .out files
to see what behavior is being tested (especially true where the output
file is a sequence of {'return': {}}).

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191114213415.23499-4-eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>