Fix AttributeErrors in nic methods

[dwettstein]

Fix a possible AttributeError in the following methods when NetworkConnection is not available:

• vm.py:delete_nic()
• vm.py:update_nic()

Additionally, I applied the same variable names in update_nic() method as used in other nic methods and fixed the error message when nic was not found.

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This change is 

[dwettstein]

What do you think about this PR, @rocknes ?

[dwettstein]

Hey @Anirudh9794 or @rocknes

I'd like to kindly ask you to have a look at this PR, as these bugs are still not fixed.

Regarding the changes in update_nic(). There are actually not many:

• I renamed to variable and logic from nic_found to nic_not_found as also used in delete_nic(),
• I renamed to loop variable nc to network as also used in delete_nic(),
• I have improved the error message in update_nic() analogous to the one used in delete_nic(),
• I moved the method update_nic() to the other nic methods.

Do you have any questions?

[dwettstein]

Hi @rocknes

Anything else I can do to get this PR merged?