Shared logic between XE and XR scripts where possible

[glennmatthews]

Currently the iosxr_iso2vbox.py and iosxe_iso2vbox.py scripts have a lot of duplicate code. So that each script can benefit from bugfixes and enhancements to the other, we should create a shared library for any code that can reasonably be shared between the two scripts.

[xiaopeng163]

agree, also for #23, it's better if this tool can be provided as a python package.