change get_image_size return type to int64_t

change get_image_size return type to int64_t

Previously, if the size of initrd >=2G, qemu exits with error:
root@haswell-OptiPlex-9020:/home/lizj# /home/lizhijian/lkp/qemu-colo/x86_64-softmmu/qemu-system-x86_64 -kernel ./vmlinuz-4.16.0-rc4 -initrd large.cgz -nographic
qemu: error reading initrd large.cgz: No such file or directory
root@haswell-OptiPlex-9020:/home/lizj# du -sh large.cgz
2.5G	large.cgz

this patch changes the caller side that use this function to calculate
size of initrd file as well.

v2: update error message and int64_t printing format

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Message-Id: <1536833233-14121-1-git-send-email-lizhijian@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>