Add ability to use LVM volumes for VM disks.
Implements LVM disks support for libvirt driver.
VM disks will be stored on LVM volumes in volume group
specified by `libvirt_images_volume_group` option.
Another option `libvirt_local_images_type` specify which storage
type will be used. Supported values are `raw`, `lvm`, `qcow2`,
`default`. If `libvirt_local_images_type` = `default`, usual
logic with `use_cow_images` flag is used.
Boolean option `libvirt_sparse_logical_volumes` controls which type
of logical volumes will be created (sparsed with virtualsize or
usual logical volumes with full space allocation). Default value
for this option is `False`.
Commit introduce three classes: `Raw`, `Qcow2` and `Lvm`. They contain
image creation logic, that was stored in
`LibvirtConnection._cache_image` and `libvirt_info` methods,
that produce right `LibvirtGuestConfigDisk` configurations for
libvirt. `Backend` class choose which image type to use.
Change-Id: I0d01cb7d2fd67de2565b8d45d34f7846ad4112c2
1fe1f66428