info does not recognize file format of vpc with subformat=fixed

After creating or converting an image to vpc with 'subformat=fixed'
'qemu-img info' incorrectly identifies the image as 'raw' format.

$ qemu-img --version
qemu-img version 2.11.1(Debian 1:2.11+dfsg-1ubuntu7.10)
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers

$ qemu-img create -f vpc -o subformat=fixed my.vpc 2G
Formatting 'my.vpc', fmt=vpc size=2147483648 subformat=fixed

$ qemu-img info my.vpc
image: my.vpc
file format: raw
virtual size: 2.0G (2147992064 bytes)
disk size: 4.0K

$ qemu-img info -f vpc my.vpc
image: my.vpc
file format: vpc
virtual size: 2.0G (2147991552 bytes)
disk size: 4.0K