trace: enable tracing in qemu-img

trace: enable tracing in qemu-img

The command will work this way:
    qemu-img --trace "qcow2*" create -f qcow2 1.img 64G

[Quote "qcow2*" to protect against shell globbing as suggested by Eric
Blake <[email protected]>.
--Stefan]

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1466174654[email protected]
Suggested by: Daniel P. Berrange <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
CC: Paolo Bonzini <[email protected]>
CC: Kevin Wolf <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>