Implement virDomainSendProcessSignal for LXC driver

Implement virDomainSendProcessSignal for LXC driver

Implement the new API for sending signals to processes in a guest
for the LXC driver. Only support sending signals to the init
process for now, because

 - The kernel does not appear to expose the mapping between
   container PID numbers and host PID numbers anywhere in the
   host OS namespace
 - There is no race-free way to validate whether a host PID
   corresponds to a process in a container.

* src/lxc/lxc_driver.c: Allow sending processes signals

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>