vvfat: correctly propagate errors

Before:
    $ ./qemu-io-old
    qemu-io-old> open -r -o driver=vvfat,fat-type=24,dir=i386-softmmu
    Valid FAT types are only 12, 16 and 32
    qemu-io-old: can't open device (null): Could not open image: Invalid argument

After:
    $ ./qemu-io
    qemu-io> open -r -o driver=vvfat,fat-type=24,dir=i386-softmmu
    qemu-io: can't open device (null): Valid FAT types are only 12, 16 and 32

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
