Introduce a new API in libvirt-qemu.so

 virDomainPtr virDomainQemuAttach(virConnectPtr domain,
                                  unsigned long long pid,
                                  unsigned int flags);

This allows libvirtd to attach to an existing, externally
launched QEMU process. This is useful for QEMU developers who
prefer to launch QEMU themselves for debugging/devel reasons,
but still want the benefit of libvirt based tools like
virt-top, virt-viewer, etc

* include/libvirt/libvirt-qemu.h: Define virDomainQemuAttach
* src/driver.h, src/libvirt-qemu.c, src/libvirt_qemu.syms:
  Driver glue for virDomainQemuAttach