run: Support launching a terminal locally

It is convenient to automatically spawn xterm when running multiple
interactive software, for example a CCA host and guest. The FVP already
supports this (when using the 'xterm' terminal type), and it is possible
with the 'telnet' terminal type but requires manually launching the
process.

Add a new 'term' type which takes an additional 'command' parameter and
is launched locally when the run starts.

An important security consideration is that this new option allows
launching arbitrary commands locally rather than sandboxed in a
container. But that was already possible by adding 'terminal_command'
parameters to the FVP. The user must trust the source of its
configurations.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>