utils: rename strtosz to use qemu prefix

Not only it makes sense, but it gets rid of checkpatch warning:
WARNING: consider using qemu_strtosz in preference to strtosz

Also remove get rid of tabs to please checkpatch.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1442419377-9309-1-git-send-email-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>