qemu: Move rng model validation to domcaps

qemu: Move rng model validation to domcaps

The qemu driver already does some <rng> model validation, based on
qemuCaps. However, the logic for exposing <rng> model values in domcaps
is basically identical. This drops the qemuCaps checking and compares
against the domCaps data directly.

This approach makes it basically impossible to add a new <rng> model to
the qemu driver without extending domcaps. The validation can also
be shared with other drivers eventually.

Reviewed-by: Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>