virsh: remove driver check from attach-disk command

virsh: remove driver check from attach-disk command

Virsh shouldn't check for driver support but rather let the backend handled this.
After removing the check, I can successfully attach file-based images to a qemu
VM with attach-disk.

% virsh attach-disk vm2 /images/test02.img vdc --driver qemu --type disk --subdriver raw
Disk attached successfully

This command generates the following XML:

<disk type='block' device='disk'>
      <driver name='qemu' type='raw'/>
      <source dev='/images/test02.img'/>
      <target dev='vdc' bus='virtio'/>
      <alias name='virtio-disk2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
</disk>

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>