libvirt_vm: Override __setattr__ of libvirt_vm.VM.

For VMs in different uri, we need some different params for them.
This patch introduce a way we can set these params in base.cfg only
for VM in a special uri. What we need to do is adding the special
param with a prefix of 'libvirt_URI_' into base.cfg. That's all.
libvirt_vm.VM will check the connect_uri and use the param you want.

Example:
	isa_serial = "serial0" is not usable for lxc VM. lxc does
not support serial device yet. In this case, we need not to change
the default value of isa_serial. We can add a new param in base.cfg.
	libvirt_lxc_isa_serial = "console0"
Then if your VM is a lxc guest, it will use "console0".

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>