build-sys: Set PKG_NAME in autogen.sh

This makes for a nicer message after running ./autogen.sh:
"Now type `make' to compile govirt"
instead of
"Now type `make' to compile Package"
cdrom: Allow empty filename when serializing to XML

Removing a CDROM image from a VM is achieved by updating the current VM
file to be "". The code currently forbid NULL filenames in update
requests, this commit removes this limitation so that it's possible to
remove a CD image from a VM through libgovirt.
resource: Fix leak of OvirtRestCall in async methods

ovirt_rest_call_async() will take its own reference on the call it
operates on, so ovirt_resource_update_async() and
ovirt_resource_invoke_action_async() need to drop their reference when
they no longer need the call they created.
Implement ovirt_resource_delete{,_finish,_async}

This will send a DELETE request to the remote resource. This can be used
to remove the image contained in a CDRom for example.