target/riscv: convert profile CPU models to RISCVCPUDef

target/riscv: convert profile CPU models to RISCVCPUDef

Profile CPUs reuse the instance_init function for bare CPUs; make them
proper subclasses instead.  Enabling a profile is now done based on the
RISCVCPUDef struct: even though there is room for only one in RISCVCPUDef,
subclasses check that the parent class's profile is enabled through the
parent profile mechanism.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>