scripts: qmp-shell: refactor helpers

Refactor the qmp-shell command line processing function
into two components. This will be used to allow sub-expressions,
which will assist us in adding transactional support to qmp-shell.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>