dma_blk_cb leaks memory map handles on misaligned IO

Since upgrading to QEMU 2.8.0, my Windows 7 64-bit virtual machines
started crashing due to the assertion quoted in the summary failing.
The assertion in question was added by commit 9972354856 ("block: add
BDS field to count in-flight requests").  My tests show that setting
discard=unmap is needed to reproduce the issue.  Speaking of
reproduction, it is a bit flaky, because I have been unable to come up
with specific instructions that would allow the issue to be triggered
outside of my environment, but I do have a semi-sane way of testing that
appears to depend on a specific initial state of data on the underlying
storage volume, actions taken within the VM and waiting for about 20
minutes.

Here is the shortest QEMU command line that I managed to reproduce the
bug with:

    qemu-system-x86_64 \
        -machine pc-i440fx-2.7,accel=kvm \
        -m 3072 \
        -drive file=/dev/lvm/qemu,format=raw,if=ide,discard=unmap \
	-netdev tap,id=hostnet0,ifname=tap0,script=no,downscript=no,vhost=on \
        -device virtio-net-pci,netdev=hostnet0 \
	-vnc :0

The underlying storage (/dev/lvm/qemu) is a thin LVM snapshot.

QEMU was compiled using:

    ./configure --python=/usr/bin/python2.7 --target-list=x86_64-softmmu
    make -j3

My virtualization environment is not really a critical one and
reproduction is not that much of a hassle, so if you need me to gather
further diagnostic information or test patches, I will be happy to help.