src/cpu: add a basic RiscV64 cpu driver

src/cpu: add a basic RiscV64 cpu driver

There are tests in qemuxml2argvtest that will fail if we enable RISC-V
testing, with an error like the following:

"cpuGetSubDriver:64 : this function is not supported by the connection
driver: 'riscv64' architecture is not supp orted by CPU driver"

This happens because we don't have a RISC-V driver yet.

Add a barebone RISC-V driver to allow tests to be executed. The only 2
callbacks implemented here are 'compare' and 'validateFeatures', both
acting as a no-op. More callbacks and features will be added in the
future.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>