tools: virsh: Reset error when keepalive registration fails

tools: virsh: Reset error when keepalive registration fails

We try to enable keepalive oportunistically. If it's not supported by
the connection driver and it was not explicitly requested we keep the
error object set and can report it in some cases accidentally:

 --- stdout ---
 TEST: /home/pipo/libvirt/tests/virsh-self-test
       !                                        1   FAILED
 --- stderr ---
 error: parameter 'target' of command 'attach-disk' must be listed before optional parameters
 error: this function is not supported by the connection driver: virConnectSetKeepAlive
 -------

Clear the stored libvirt error.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>