vm/adb: Use the correct path for debugfs

[sebastianene07]

Probe for the debugfs rootdir instead of the kcov
sub-path to prevent the fuzzer from entering in
device reboot loop in case the android device
doesn't support kcov.

Fixes: #6600

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[a-nogikh]

Please address the following failed check:
Error: Wrong commit subject format: 'vm/adb: Use the correct path for debugfs'. Please use 'main/affected/package: short change description'. See docs/contributing.md for details. Regex pattern used to check commit subjects is '^(Revert "|(([a-z0-9/_.-]+|Makefile|AUTHORS|CONTRIBUTORS|README.md)(, )?)+:\ [^A-Z].+[^.]$)'.

[a-nogikh]

Thanks!