The legacy resolver was enabled when the new pip was released.
The requirements have been cleaned up, and sysinv has been
pkgconfig is required to be able to install libvirt from pip.
A bindep file has been added to ensure the components required
to build libvirt-python are available on the zuul host.
The upper constraints have been removed from tox/zuul since
the headers for components installed by bindep are newer than
what will work when compiling the older libvirt
Signed-off-by: albailey <Al.Bailey@windriver.com>