KVM: selftests: Return an 'unsigned int' from kvm_check_cap()

Return an 'unsigned int' instead of a signed 'int' from kvm_check_cap(),
to make it more obvious that kvm_check_cap() can never return a negative
value due to its assertion that the return is ">= 0".

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>