lxd/device/disk: Pass cloud-init:config drive to QEMU using file descriptor
lxd/device/disk: Pass cloud-init:config drive to QEMU using file descriptor

Aligns with how other disk device types are passed to QEMU.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd