target/riscv/kvm: support KVM_GET_REG_LIST

KVM for RISC-V started supporting KVM_GET_REG_LIST in Linux 6.6. It
consists of a KVM ioctl() that retrieves a list of all available regs
for get_one_reg/set_one_reg. Regs that aren't present in the list aren't
supported in the host.

This simplifies our lives when initing the KVM regs since we don't have
to always attempt a KVM_GET_ONE_REG for all regs QEMU knows. We'll only
attempt a get_one_reg() if we're sure the reg is supported, i.e. it was
retrieved by KVM_GET_REG_LIST. Any error in get_one_reg() will then
always considered fatal, instead of having to handle special error codes
that might indicate a non-fatal failure.

Start by moving the current kvm_riscv_init_multiext_cfg() logic into a
new kvm_riscv_read_multiext_legacy() helper. We'll prioritize using
KVM_GET_REG_LIST, so check if we have it available and, in case we
don't, use the legacy() logic.

Otherwise, retrieve the available reg list and use it to check if the
host supports our known KVM regs, doing the usual get_one_reg() for
the supported regs and setting cpu->cfg accordingly.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20231003132148.797921-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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