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