doc/bdev: add configuration info for GPT

~~~

This exports 1 rbd block device, named Ceph0.

## GPT (GUID Partition Table) {#bdev_config_gpt}

The GPT virtual bdev driver examines all bdevs as they are added and exposes partitions

with a SPDK-specific partition type as bdevs.

The SPDK partition type GUID is `7c5222bd-8f5d-4087-9c00-bf9843c7b58c`.

Configuration file syntax:

~~~

[Gpt]

  # If Gpt is disabled, it will not automatically expose GPT partitions as bdevs.

  Disable No

~~~

### Creating a GPT partition table using NBD

The bdev NBD app can be used to temporarily expose an SPDK bdev through the Linux kernel

block stack so that standard partitioning tools can be used.

~~~

# Expose bdev Nvme0n1 as kernel block device /dev/nbd0

# Assumes bdev.conf is already configured with a bdev named Nvme0n1 -

# see the NVMe section above.

test/lib/bdev/nbd/nbd -c bdev.conf -b Nvme0n1 -n /dev/nbd0 &

nbd_pid=$!

# Create GPT partition table.

parted -s /dev/nbd0 mklabel gpt

# Add a partition consuming 50% of the available space.

parted -s /dev/nbd0 mkpart MyPartition '0%' '50%'

# Change the partition type to the SPDK GUID.

# sgdisk is part of the gdisk package.

sgdisk -t 1:7c5222bd-8f5d-4087-9c00-bf9843c7b58c /dev/nbd0

# Kill the NBD application (stop exporting /dev/nbd0).

kill $nbd_pid

# Now Nvme0n1 is configured with a GPT partition table, and

# the first partition will be automatically exposed as

# Nvme0n1p1 in SPDK applications.

~~~