Move some libvirt specific flags into a group

This is backwards compatible via deprecated groups. This change
moves the flags defined in nova/virt/libvirt/driver.py into a
"libvirt" configuration group.

There are other flags which need to be moved, but this change
is quite large, so I will handle those in follow up changes.

Now that the flags in nova/virt/libvirt/driver.py are in a libvirt
group, we can drop the libvirt_ from in front of many of the flag
names as well.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags are moved into a "libvirt" flag
group: rescue_image_id, rescue_kernel_id, rescue_ramdisk_id,
libvirt_type, libvirt_uri, libvirt_inject_password,
libvirt_inject_key, libvirt_inject_partition, use_usb_tablet,
live_migration_uri, live_migration_flag, block_migration_flag,
live_migration_bandwidth, snapshot_image_format,
libvirt_vif_driver, libvirt_volume_drivers

The following flags have had the libvirt_ dropped from the start of
their flag name now that they are in a flag group:
    libvirt_type (now virt_type)
    libvirt_uri (now conn_uri)
    libvirt_inject_password
    libvirt_inject_key
    libvirt_inject_partition
    libvirt_vif_driver
    libvirt_volume_drivers
    libvirt_disk_prefix
    libvirt_wait_soft_reboot_seconds
    libvirt_non_blocking
    libvirt_cpu_mode
    libvirt_cpu_model
    libvirt_snapshots_directory

The name for the virt_type flag was influenced by bug 1195361,
which shows confusion as to what the flag is setting (the
virtualization type libvirt uses, not the driver libvirt uses
to achieve that).

Finally, the flag libvirt.nonblocking has been renamed to
libvirt.api_thread_pool, which is more descriptive.

Change-Id: I11e79eb5fd79c459374b9bf04707a4e6d79e993f
Merge "Move some libvirt specific flags into a group"