qemu-doc: Fix ivshmem huge page example

Option parameter "share" is missing.  Without it, you get a *private*
mmap(), which defeats ivshmem's purpose pretty thoroughly ;)

While there, switch to the conventional mountpoint of hugetlbfs
/dev/hugepages.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1458066895-20632-5-git-send-email-armbru@redhat.com>

qemu-doc.texi

@@ -1311,7 +1311,7 @@ Instead of specifying the <shm size> using POSIX shm, you may specify
 a memory backend that has hugepage support:
 
 @example
-qemu-system-i386 -object memory-backend-file,size=1G,mem-path=/mnt/hugepages/my-shmem-file,id=mb1
+qemu-system-i386 -object memory-backend-file,size=1G,mem-path=/dev/hugepages/my-shmem-file,share,id=mb1
                  -device ivshmem,x-memdev=mb1
 @end example