lxd/storage/drivers/driver/btrfs/volumes: Create temporary snapshot in BackupVolume()
lxd/storage/drivers/driver/btrfs/volumes: Create temporary snapshot in BackupVolume()

Ensures consistent backups when instance files are being modified during backup.

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

lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,lxc/lxd,Skarlso/lxd,Skarlso/lxd,Skarlso/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,Skarlso/lxd,hallyn/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,stgraber/lxd,mjeanson/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd