kvm_geterr_test: Compile out the portions that require kvm_open2(3) on
systems that lack the libcall, based on __FreeBSD_version.

kvm_open2(3) wasn't made available until r291406, which is in ^/stable/11,
but not ^/stable/10. This makes some of kvm_geterr_test available for testing
on ^/stable/10.

MFC after: now
Sponsored by: Dell EMC Isilon