selftests: kvm: return Kselftest Skip code for skipped tests

When kvm test is skipped because of unmet dependencies and/or unsupported
configuration, it exits with error which is treated as a fail by the
Kselftest framework. This leads to false negative result even when the test
could not be run.

Change it to return kselftest skip code when a test gets skipped to clearly
report that the test could not be run.

Change it to use ksft_exit_skip() when the test is skipped. In addition,
refine test_assert() message to include strerror() string and add explicit
check for EACCES to cleary identify when test doesn't run when access is
denied to resources required e.g: open /dev/kvm failed, rc: -1 errno: 13

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>