RISC-V: KVM: Probe for SBI extension status

Rather than defaulting the status to available and allowing the user
to set availability, default to uninitialized and only allow the user
to set the status to unavailable. Then, when an extension is first
used, ensure it is available by invoking its probe function, if it
has one (an extension is assumed available if it doesn't have a probe
function). Checking the status in kvm_vcpu_sbi_find_ext() ensures
extension functions cannot be invoked when they're unavailable.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>