qemu: Utilize virFileFindHugeTLBFS

qemu: Utilize virFileFindHugeTLBFS

Use better detection of hugetlbfs mount points. Yes, there can be
multiple mount points each serving different huge page size.

Since we already have ability to override the mount point in the
qemu.conf file, this crazy backward compatibility code is brought in.
Now we allow multiple mount points, so the "hugetlbfs_mount" option
must take an list of strings (mount points). But previously, it was
just a string, so we must accept both types now.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>