target/riscv: use KVM scratch CPUs to init KVM properties

Certain validations, such as the validations done for the machine IDs
(mvendorid/marchid/mimpid), are done before starting the CPU.
Non-dynamic (named) CPUs tries to match user input with a preset
default. As it is today we can't prefetch a KVM default for these cases
because we're only able to read/write KVM regs after the vcpu is
spinning.

Our target/arm friends use a concept called "scratch CPU", which
consists of creating a vcpu for doing queries and validations and so on,
which is discarded shortly after use [1]. This is a suitable solution
for what we need so let's implement it in target/riscv as well.

kvm_riscv_init_machine_ids() will be used to do any pre-launch setup for
KVM CPUs, via riscv_cpu_add_user_properties(). The function will create
a KVM scratch CPU, fetch KVM regs that work as default values for user
properties, and then discard the scratch CPU afterwards.

We're starting by initializing 'mvendorid'. This concept will be used to
init other KVM specific properties in the next patches as well.

[1] target/arm/kvm.c, kvm_arm_create_scratch_host_vcpu()

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
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: <20230706101738.460804-7-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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