selftests: KVM: skip hugetlb tests if huge pages are not available

Right now, if KVM memory stress tests are run with hugetlb sources but hugetlb is
not available (either in the kernel or because /proc/sys/vm/nr_hugepages is 0)
the test will fail with a memory allocation error.

This makes it impossible to add tests that default to hugetlb-backed memory,
because on a machine with a default configuration they will fail.  Therefore,
check HugePages_Total as well and, if zero, direct the user to enable hugepages
in procfs.  Furthermore, return KSFT_SKIP whenever hugetlb is not available.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>