aemfvp-a-rme: support network connection to fvp in boot test

boot.sh use case booted an aemfvp-a-rme fvp until login, this change adds
support of network connection to FVP via a tap interface that is setup at
test machine and used as model param for network connectivity. Updates are
- find_tap_interface to look for available tap interfaces available at
  host machine.
- NTW_ENABLE introduced in run_model to indicate connection request.
- enabling model params for virtio_net and hostbridge.interface=tap to
  allow tap interface connection to FVP.
- add -n <true/false> param in boot.sh to enable network.
- add model params for pcie to provide an image for boot.
- virtio block device param to FVP added to attach a satadisk.

Signed-off-by: nitin sharma <nitin.sharma3@arm.com>
Change-Id: I196c1b4c66bd19c213b2fd389fb47ef1a4e287c6