QEMU has -fw_cfg which allows users to tweak how firmware
configures itself and/or provide new configuration blobs.
Introduce new <sysinfo/> type "fwcfg" that will hold these
new blobs.

It's possible to either specify new value as a string or
provide a filename which contents then serve as the value.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>