Add public API to register a callback to be invoked on connection close

Add public API to register a callback to be invoked on connection close

Define new virConnect{Register,Unregister}CloseCallback() public APIs
which allows registering/unregistering a callback to be invoked when
the connection to a hypervisor is closed. The callback is provided
with the reason for the close, which may be 'error', 'eof', 'client'
or 'keepalive'.

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