CLOUDSTACK-6191: New feature: Volume provisioning type option

Make disk provisioning type configurable from disk|service offering
Make disk|service offering API return a provisioning type
Add UI to display and configure provisioning type of a volume
Add preallocation column to the volumes table and disk_offering table
Add provisioning type to volume_view, disk_offering_view, and service_offering_view
Set preallocation option as qemu-img option
Change qemu-img info output format from human to json to get acurate disk size
Replace libvirt by qemu-img for volume creation to support preallocation option
Add a test case to test qemu-img with preallocation option
Add test cases for the provisioning type parameter of a disk offering
Refactor LibVirtStorageAdaptor#createPhysicalDisk
Refactor LibvirtStorageAdoptor#createDiskFromTemplate to shape up