drive_mirror on a resized image creates file with wrong size

Repro steps:

qemu-img create -f qcow2 base 32M
qemu-img create -f qcow2 -o backing_file=base disk
qemu-img resize /home/vishvananda/disk 64M
qemu-system-x86_64 -drive file=disk,id=vda -vnc :1 -monitor stdio
QEMU 1.3.0 monitor - type 'help' for more information
(qemu) drive_mirror vda test
Formatting 'test', fmt=qcow2 size=33554432 backing_file='base' backing_fmt='qcow2' encryption=off cluster_size=65536 lazy_refcounts=off

the file is 32M instead of 64M:

qemu-img info test
image: test
file format: qcow2
virtual size: 32M (33554432 bytes)
disk size: 196K
cluster_size: 65536
backing file: base
backing file format: qcow2