KVM: selftests: Assert that __vm_get_stat() actually finds a stat

KVM: selftests: Assert that __vm_get_stat() actually finds a stat

Fail the test if it attempts to read a stat that doesn't exist, e.g. due
to a typo (hooray, strings), or because the test tried to get a stat for
the wrong scope.  As is, there's no indiciation of failure and @data is
left untouched, e.g. holds '0' or random stack data in most cases.

Fixes: 8448ec5993be ("KVM: selftests: Add NX huge pages test")
Link: https://lore.kernel.org/r/20250111005049.1247555-4-seanjc@google.com
[sean: fixup spelling mistake, courtesy of Colin Ian King]
Signed-off-by: Sean Christopherson <seanjc@google.com>