riscv/virt: Support using pflash via -blockdev option

riscv/virt: Support using pflash via -blockdev option

Currently, pflash devices can be configured only via -pflash
or -drive options. This is the legacy way and the
better way is to use -blockdev as in other architectures.
libvirt also has moved to use -blockdev method.

To support -blockdev option, pflash devices need to be
created in instance_init itself. So, update the code to
move the virt_flash_create() to instance_init. Also, use
standard interfaces to detect whether pflash0 is
configured or not.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reported-by: Andrea Bolognani <abologna@redhat.com>
Tested-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230601045910.18646-3-sunilvl@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>