mach-virt: Set VM's SMBIOS system version to mc->name

Instead of using "1.0" as the system version of SMBIOS, we should use
mc->name for mach-virt machine type to be consistent other architectures.
With this patch, "dmidecode -t 1" (e.g., "-M virt-2.12,accel=kvm") will
show:

    Handle 0x0100, DMI type 1, 27 bytes
    System Information
            Manufacturer: QEMU
            Product Name: KVM Virtual Machine
            Version: virt-2.12
            Serial Number: Not Specified
            ...

instead of:

    Handle 0x0100, DMI type 1, 27 bytes
    System Information
            Manufacturer: QEMU
            Product Name: KVM Virtual Machine
            Version: 1.0
            Serial Number: Not Specified
            ...

For backward compatibility, we allow older machine types to keep "1.0"
as the default system version.

Signed-off-by: Wei Huang <wei@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20180322212318.7182-1-wei@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>