qmp-shell: add persistent command history

qmp-shell: add persistent command history

Use the existing readline history function we are utilizing
to provide persistent command history across instances of qmp-shell.

This assists entering debug commands across sessions that may be
interrupted by QEMU sessions terminating, where the qmp-shell has
to be relaunched.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20170427223628.20893-1-jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>