qemu: process: Introduce setup of block-device backed NVRAM

qemu: process: Introduce setup of block-device backed NVRAM

In case when a management application will require to store the nvram in
a block device instead of a file libvirt needs to be able to set up the
block device.

This patch introduces support for setting up the block device by using
'qemu-img convert' to produce a qcow2-formatted block device.

The use of 'qcow2' is made mandatory as the UEFI firmware requires that
the NVRAM image has the exact expected size, which is almost impossible
with block devices. 'qcow2' also allows libvirt to detect wheher the
block device is formatted allowing file-like semantics.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>