libvirt: use tap for non-blockdevice images on Xen

This patch reverts 35c4962c0b97bae5b8751d316d5822fe22c1ab6a: 'use
file instead of tap for non-blockdevice images on Xen', which breaks
qcow2 disk working on Xen. If the method pick_disk_driver_name()
returns 'file' for qcow2 format, we would get the following disk XML
snippet:

    <disk type="file" device="disk">
      <driver name="file" type="qcow2" cache="none"/>
      <source file="/data/instances/instance-00000081/disk"/>
      <target bus="xen" dev="hda"/>
    </disk>

which produces configuration in the xen tools like:

    "file:/data/instances/instance-00000081/disk,hda,w"

Guest would not boot successfully, as Xen doesn't think of disk format
as qcow2. Instead, 'tap' works with both raw and qcow2 disk.

For LP#1084618, I think it is related with libvirt verion. In my
deployment(libvirt-1.0.1), 'tap' dirver could produce correct
configuration for both raw and qcow2 disk format.

Change-Id: I636b2fed366474e2ae8c3b52403e3085724b24f0
Don't warn up front about libvirt loading issues in NWFilterFirewall

Use the same technique as in libvirt/driver.py to delay import of
libvirt module

Change-Id: I500ddde4651f0f747c54533a1a308eee5a87f717
Merge "libvirt: use tap for non-blockdevice images on Xen"
Merge "Don't warn up front about libvirt loading issues in NWFilterFirewall"