qemu: agent: Store CCW address in qemuAgentDiskInfo if provided by the guest

qemu: agent: Store CCW address in qemuAgentDiskInfo if provided by the guest

Newer versions of the QEMU guest agent will provide the CCW address
of devices on s390x. Store this information in the qemuAgentDiskInfo
so that we can use this later.

We also map the CSSID 0 from the guest to the value 0xfe on the host,
see https://www.qemu.org/docs/master/system/s390x/css.html for details.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>