hw/virtio-9p-local.c: Remove unnecessary null char in symlink file

This patch removes the addition of null char in symlink file
which is being appended to file in case of mapped security model.
Without this patch, the extra null char causes LTP testcase lstat03
to fail and hence this fix is required.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
