Do not generate security_model when fs driver is anything but 'path'

Do not generate security_model when fs driver is anything but 'path'

QEMU does not support security_model for anything but 'path' fs driver type.
Currently in libvirt, when security_model ( accessmode attribute) is not
specified it auto-generates it irrespective of the fs driver type, which
can result in a qemu error for drivers other than path. This patch ensures
that the qemu cmdline is correctly generated by taking into account the
fs driver type.

Signed-off-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>