Replace the string concatenation in the monitor command line construction

[cmainas]

In the vast majority of urunc's monitor support, we make use of inefficient string concatenation. In particular the pattern cli += " option" is very common. While this patter is easy to read, it is inefficient and at the end we do not really need a big long string for the cli options of each monitor. Instead we need a slice of strings where each string is one option. Therefore, we need to update the construction of cli options in all monitors and use a much more efficient way.

[singhshresth26]

I have addressed this issue in PR #858.

The Unikernel interface and related structures have been updated to return and use []string directly. Additionally, all hypervisors have been refactored to construct their command-line arguments as slices directly, eliminating the inefficient string concatenation pattern and subsequent strings.Split calls.