virhostmem: Introduce virHostMemGetTHPSize()

virhostmem: Introduce virHostMemGetTHPSize()

New virHostMemGetTHPSize() is introduced which allows caller to
obtain THP PMD (Page Middle Directory) size, which is equal to
the minimal size that THP can use, taken from kernel doc
(Documentation/admin-guide/mm/transhuge.rst):

  Some userspace (such as a test program, or an optimized memory allocation
  library) may want to know the size (in bytes) of a transparent hugepage::

    cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size

Since this size depends on the host architecture and the kernel
it won't change whilst libvirtd is running. Therefore, we can use
virOnce() and cache the value. Of course, we can be running under
kernel that has THP disabled or has no notion of THP at all. In
that case a negative value is returned to signal error.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>