Can't seem to be able to select a vsys as a rule target

Describe the bug

It seems to be impossible to select a vsys as a rule target when creating a new rule, maybe related to #199

Steps to reproduce

from panos import panorama, policies

p = panorama.Panorama(IP, USER, PASSWORD)
devices_groups = p.refresh_devices()
device_group = next(d for d in devices_groups if d.name == REQUIRED_DEVICE_GROUP)
pre_rulebase = policies.PreRulebase()
device_group.add(pre_rulebase)
new_rule = panorama.policies.SecurityRule(target=['001801005535/vsys2'],
                                          source=['1.1.1.1'],
                                          destination=['2.2.2.2'],
                                          service=['http'],
                                          action='allow',
                                          name='new rule 1')
pre_rulebase.add(new_rule)
new_rule.create()
p.commit(sync=True)

Results in

panos.errors.PanDeviceXapiError:  new rule 1 -> target -> devices -> 001801005535/vsys2 '001801005535/vsys2' is invalid
new rule 1 -> target -> devices is invalid

Environment

panos 1.0.0

Start with the panorama.policies.SecurityRule entry point and reproduce the failure using the provided vsys target example. Trace how the target value is validated during new-rule creation, then verify that a valid vsys target is accepted without the PanDeviceXapiError and add or run a regression test if the project provides one.