Removing Arm Virtual Hardware references from documentation

* Arm Virtual Hardware (AVH) AMI is being deprecated so
  we are removing references to this in our documentation
* Minor typo fix in FVP arguments documentation

Change-Id: Id7a7af64d44a4b5295280c5808533040932145c2
Signed-off-by: Alex Tawse <alex.tawse@arm.com>