scsi: Specify the xfer direction for UNMAP and ATA_PASSTHROUGH commands

scsi_cmd_xfer_mode() is used to specify the xfer direction for SCSI
commands that come in from the guest.  If the direction is set incorrectly
this will eventually cause QEMU to kernel-panic the guest.

Add UNMAP and ATAPASSTHROUGH as commands that send data to the device.

Without this change, recent kernels will send both UNMAP as well
as ATAPASSTHROUGH commands to any /dev/sg* device, which due to the
incorrect xfer direction very quickly causes the guest kernel to crash.

Example causing a crash without the patch applied:

./x86_64-softmmu/qemu-system-x86_64 -m 1024 -enable-kvm -cdrom linuxmint-12-gnome-dvd-64bit.iso -drive file=/dev/sg4,if=scsi,bus=0,unit=6

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
