[pdpix] Support Coexistence of Different LibOS Types

[ppenna]

Context

Currently, Demikernel does not support multiple LibOSes, each of which of a different type, to co-exist.

This may be limiting when an application interacts with multiple kernel-bypass technologies at the same time.

Proposed Solution

In order to enable multiple LibOSes to co-exist, we shall work on the following steps:

• [x] Enable compilation of LibOSes to be composed (d61ae7bdf605d1c032c30e809fcafcf04ffe5798)
• [x] Enable dynamic selection of which LibOSes flavor should be instantiated (d61ae7bdf605d1c032c30e809fcafcf04ffe5798)
• [ ] Enable instantiation of multiple LibOSes.