Enable support of multipath disks for VMs created by fuel-devops

fuel-devops should be able to configure the multipath disk devices on guest VMs.

It is required for testing the feature [1].

To emulate the multipath disk device on libvirt VMs, fuel-devops should connect the same volume miltiple times as a different disk devices, for example:

<devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source dev='/dev/Linux/kvm-tmp'/>
      <target dev='sda' bus='scsi'/>
      <serial>0001</serial>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source dev='/dev/Linux/kvm-tmp'/>
      <target dev='sdb' bus='scsi'/>
      <serial>0001</serial>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

[1] https://blueprints.launchpad.net/fuel/+spec/multipath-disks-support