Use "ip link show dev xx" to check whether link existed in vm detach_nic callback
Use "ip link show dev xx" to check whether link existed in vm detach_nic callback

Fix ZSTAC-8873

zstackio/zstack-utility,live4thee/zstack-utility,zstackio/zstack-utility,zstackio/zstack-utility,zstackorg/zstack-utility,zstackorg/zstack-utility,live4thee/zstack-utility,zstackorg/zstack-utility,live4thee/zstack-utility