boot_hostdev vdsm hook
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This hook allows to boot VMs from passthrough devices such as SCSI, PCI cards
and USB devices.

Installation:
* Use engine-config to set the appropriate custom properties as such:

Custom property for all interfaces in a VM:
    sudo engine-config -s "UserDefinedVMProperties=boot_hostdev=^.*$"

Usage:
1. Pass a device to a VM using the Host Devices tab.
2. Copy the device name
3. Edit the VM, in custom properties set boot_hostdev and paste the device name
4. Start the VM

The hook will set the hostdev from step 2 with bootindex=1 in qemu command line,
all other devices will have their boot indexes bumped by 1.
