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

Add the library entry point for the new virDomainQemuMonitorCommand()
entry point.  Because this is not part of the "normal" libvirt API,
it gets its own header file, library file, and will eventually
get its own over-the-wire protocol later in the series.

Changes since v1:
 - Go back to using the virDriver table for qemuDomainMonitorCommand, due to
   linking issues
 - Added versioning information to the libvirt-qemu.so

Changes since v2:
 - None

Changes since v3:
 - Add LGPL header to libvirt-qemu.c
 - Make virLibConnError and virLibDomainError macros instead of function calls

Changes since v4:
 - Move exported symbols to libvirt_qemu.syms
Signed-off-by: NChris Lalancette <clalance@redhat.com>