libvirt.tests: Use sync instead of directly setting for recovery.

On older libvirt, defining an exist network will cause failure,
but for latest libvirt, it allows to define exist network if
this network is transient.
Function sync(...) can solve this problem for both versions.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #262 from yumingfei/fix_virsh_net

libvirt.tests: Fix network persistent bug.