release PCI address only when we have ensured it successfully

release PCI address only when we have ensured it successfully

Steps to reproduce this bug:
1. # cat net.xml # 00:03.0 has been used
    <interface type='network'>
      <mac address='52:54:00:04:72:f3'/>
      <source network='default'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

2. # virsh attach-device vm1 net.xml
   error: Failed to attach device from net.xml
   error: internal error unable to reserve PCI address 0:0:3

3. # virsh attach-device vm1 net.xml
   error: Failed to attach device from net.xml
   error: internal error unable to execute QEMU command 'device_add': Device 'rtl8139' could not be initialized

The reason of this bug is that: we can not reserve PCI address 0:0:3 because it has
been used, but we release PCI address when we reserve it failed.