virsysinfo: Try reading DMI table

virsysinfo: Try reading DMI table

Add DMI support for risc-v and mips. Attempt to read dmidecode and
fall back to old behavior if that fails.

The SMBIOS specification[1] officially supports both RISC-V and LoongArch.
Some mips-based Loongson-3 processors also have SMBIOS.

[1] https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.7.0.pdf

Signed-off-by: Brett Holman <brett.holman@canonical.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>