qcow2: Add falloc and full preallocation option

This adds preallocation=falloc and preallocation=full mode to qcow2
image creation.

preallocation=full allocates disk space by writing zeros to disk to
ensure disk space in any cases.

preallocation=falloc likes preallocation=full, but allocates disk space
by posix_fallocate().

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>