target/riscv/kvm.c: update KVM MISA bits

Our design philosophy with KVM properties can be resumed in two main
decisions based on KVM interface availability and what the user wants to
do:

- if the user disables an extension that the host KVM module doesn't
know about (i.e. it doesn't implement the kvm_get_one_reg() interface),
keep booting the CPU. This will avoid users having to deal with issues
with older KVM versions while disabling features they don't care;

- for any other case we're going to error out immediately. If the user
wants to enable a feature that KVM doesn't know about this a problem that
is worth aborting - the user must know that the feature wasn't enabled
in the hart. Likewise, if KVM knows about the extension, the user wants
to enable/disable it, and we fail to do it so, that's also a problem we
can't shrug it off.

In the case of MISA bits we won't even try enabling bits that aren't
already available in the host. The ioctl() is so likely to fail that
it's not worth trying. This check is already done in the previous patch,
in kvm_cpu_set_misa_ext_cfg(), thus we don't need to worry about it now.

In kvm_riscv_update_cpu_misa_ext() we'll go through every potential user
option and do as follows:

- if the user didn't set the property or set to the same value of the
host, do nothing;

- Disable the given extension in KVM. Error out if anything goes wrong.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-14-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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