virtio-blk.c handling of i/o which is not a 512 multiple

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I'm trying to write a virtio-blk driver for Solaris. I've gotten it to the point
where Solaris can see the device and create a ZFS file system on it.

However when I try and create a UFS filesystem on the device, the VM crashed
with the error
*** glibc detected *** /usr/bin/qemu-kvm: double free or corruption (!prev):
0x00007f2d38000a00 ***

I can reproduce the problem with a simple dd, i.e.
dd if=/dev/zero of=/dev/rdsk/c2d10p0 bs=5000 count=1

My driver will create a virtio-blk request with two elements in the sg list, one
for the first 4096 byes and the other for the remaining 904.

>From stepping through with gdb, virtio_blk_handle_write will sets n_sectors to 9
(5000 / 512). Later on the code, n_sectors is used the calculate the size of the
buffer required but 9 * 512 is too small and so when the request is process it
ends up writing past the end of the buffer and I guest this triggers the glibc

Is there a requirement for virtio-blk guest drivers that all i/o requests are
sized in multiples of 512 bytes?

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