Fix storage volume format on logical pool for vm

VM with LV as its disk will fail to start.
Root cause is volume format is set to be qcow2,
space of which is not allocated at creation time, just with metadata.
Data of files can be allocated through writes.
But LV cannot extend accordingly as file unless manually call 'lvextend'.
So create logical volume formated as 'raw' to allocate enough space,
This will prevent vm write access exceeds range.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
logical pool: Fix logical pool target path

Logical pool target path, which refers to the VG path,
is always under /dev.
Specify a random file system path will not take effective.
LV link is always created under /dev/VG-name/.
REF:
  libvirt/src/storage/storage_backend_logical.c
  virStorageBackendLogicalBuildPool()

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>