lxd/instance/drivers/driver/qemu: Rework addDriveConfig to support file descriptors encoded into device paths
lxd/instance/drivers/driver/qemu: Rework addDriveConfig to support file descriptors encoded into device paths

If the driveConf.DevPath is local (not rbd) then attempt to recognise an fd: prefixed encoded device path and extract
both the fd number and the original device path name.

Then use /proc/self/fd/<fdNum> to map the FD into the QEMU config.

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

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