util/mmap-alloc: qemu_fd_getfs()

This new helper fetches file system type for a fd.  Only Linux is
implemented so far.  Currently only tmpfs and hugetlbfs are defined,
but it can grow as needed.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>