Introduce krun_has_feature() API

[mtjhrc]

Introduce a function to query at runtime whether a specific feature was enabled at build time. This allows applications to check for optional capabilities before attempting to use them. This is especially useful for our tests and examples.

[mtjhrc]

Is this even a good idea? I mean an alternative could be checking if the related symbol names exists with dlysm, but we are not always constituent on whether the function exists or just returns ENOTSUP or doesn't even return any error at all e.g. krun_set_gpu_options

[tylerfanelli]

#521 changes NITRO to AWS_NITRO, perhaps that could be modified here?

[mtjhrc]

Yeah, that is nicer.

Hmm, perhaps we should we also change these for consistency?
TDX -> INTEL_TDX
SEV -> AMD_SEV

I mean we should probably change the name in the Makefile too, but this seems like a more user friendly naming convention.

[jakecorrenti]

I like this idea and it reminds me of KVM's feature/extension API.

Rather than using strings, I wonder if we should do something similar to KVM where we have something along these lines:

// features are integers exposed by a public constant
#define KRUN_FEATURE_NET 0
#define KRUN_FEATURE_BLK 1
...
#define KRUN_FEATURE_VIRGL_RESOURCE_MAP2 10

// user calls API to check feature using the constant instead of hard-coded string
int ret = krun_has_feature(KRUN_FEATURE_NET);

[mtjhrc]

Rather than using strings, I wonder if we should do something similar to KVM where we have something along these lines:

Yeah, I've changed it to use integers.