lxd/storage/drivers/generic/vfs: Updates genericVFSCopyVolume to not copy block volume files twice
lxd/storage/drivers/generic/vfs: Updates genericVFSCopyVolume to not copy block volume files twice

For filesystem backed storage pools (dir and btrfs) this function was previously copying the block
volume files twice, first via rsync and then second using Go's io.Copy().

This commit changes it do behave more like genericVFSMigrateVolume where the rsync part of the
transfer excludes the block volume file.

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

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