qemu: Validate firmware blob configuration

qemu: Validate firmware blob configuration

There are recommendations and limitations to the name of the
config blobs we need to follow [1].

We don't want users to change any value only add new blobs. This
means, that the name must have "opt/" prefix and at the same time
must not begin with "opt/ovmf" nor "opt/org.qemu" as these are
reserved for OVMF or QEMU respectively.

1: docs/specs/fw_cfg.txt from qemu.git

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>