arm/xilinx_zynq: Always instantiate the GEMs

Don't conditionalise GEM instantiation on networking attachments. The
device should always be present even if not attached to a network.

This allows for probing of the device by expectant guests (such as
OS's).  This is needed because sysbus (or AXI in Xilinx's real hw case)
is not self identifying so the guest has no dynamic way of detecting
device absence.

Also allows for testing of the GEM in loopback mode with -net none.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 55649779a68ee3ff54b24c339b6fdbdccd1f0ed7.1388800598.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
