KVM: selftests: List all hugetlb src types specified with page sizes

With VM_MEM_SRC_ANONYMOUS_HUGETLB, we currently can only use system
default hugetlb pages to back the testing guest memory. In order to
add flexibility, now list all the known hugetlb backing src types with
different page sizes, so that we can specify use of hugetlb pages of the
exact granularity that we want. And as all the known hugetlb page sizes
are listed, it's appropriate for all architectures.

Besides, the helper get_backing_src_pagesz() is added to get the
granularity of different backing src types(anonumous, thp, hugetlb).

Suggested-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20210330080856.14940-9-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>