libvirt: add a configuration file named drivers.cfg

Currently, the special configurations for specific driver is rewrite
in setting params of libvirt_vm.VM. This patch move this work to the
configuration file parsing.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt_vm: recreate serial in lxc.wait_for_login()

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #1740 from yangdongsheng/lxc

libvirt: add a configuration file named drivers.cfg