cpu_riscv64.c: add update() implementation

cpu_riscv64.c: add update() implementation

At this moment it is not possible to launch a 'riscv64' domain if a CPU
definition is presented in the domain. For example, adding this CPU
definition:

  <cpu mode='custom' match='exact' check='none'>
    <model fallback='forbid'>rv64</model>
  </cpu>

Will trigger the following error:

$ sudo ./run tools/virsh start riscv-virt1
error: Failed to start domain 'riscv-virt1'
error: this function is not supported by the connection driver:
       cannot update guest CPU for riscv64 architecture

The error comes from virCPUUpdate(), via qemuProcessUpdateGuestCPU(),
and it's caused by the absence of the 'update' API in the existing
RISC-V driver.

Add an 'update' API impl to the RISC-V driver to allow for CPU
definitions to be declared in RISC-V domains. This API was copied from
the ARM driver (virCPUarmUpdate()) since it's a good enough
implementation to get us going.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>