qemu-img: report size overflow error message

qemu-img will complain when qcow or qcow2
size overflow for 64 bits, report the right
message in this condition.

$./qemu-img create -f qcow2 /tmp/foo 0x10000000000000000
before change:
qemu-img: Invalid image size specified! You may use k, M, G or T suffixes for
qemu-img: kilobytes, megabytes, gigabytes and terabytes.

after change:
qemu-img: Image size must be less than 8 EiB!

[Resolved conflict with a9300911 goto removal -- Stefan]

Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
