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>