qemu_cmdline hook:
==================
libvirt provides support for passing QEMU cmdline options.
With the help of this, one can inject QEMU options bypassing
the libvirt's domain XML code. This can help immensely in
testing and developing new qemu features, which are not yet in
libvirt and/or injecting one-time or some-time kind of stuff :)

The os environ 'qemu_cmdline' points to a json list, containing
the qemu option followed by value as individual list entries.
For qemu option that does not take any value, skip the value entry.

Note that validation of the option and value list entries is NOT done,
its passed as-is to qemu and in the same order as present in the list.

For eg: qemu_cmdline='["-cdrom","<path/to/iso>", ...]'

