Implement code to attach to external QEMU instances.

Implement code to attach to external QEMU instances.

Given a PID, the QEMU driver reads /proc/$PID/cmdline and
/proc/$PID/environ to get the configuration. This is fed
into the ARGV->XML convertor to build an XML configuration
for the process.

/proc/$PID/exe is resolved to identify the full command
binary path

After checking for name/uuid uniqueness, an attempt is
made to connect to the monitor socket. If successful
then 'info status' and 'info kvm' are issued to determine
whether the CPUs are running and if KVM is enabled.

* src/qemu/qemu_driver.c: Implement virDomainQemuAttach
* src/qemu/qemu_process.h, src/qemu/qemu_process.c: Add
  qemuProcessAttach to connect to the monitor of an
  existing QEMU process