smbios: handle errors consistently

smbios: handle errors consistently

Current code uses mix of error_report()+exit(1)
and error_setg() to handle errors.
Use newer error_setg() everywhere, beside consistency
it will allow to detect error condition without killing
QEMU and attempt switch-over to SMBIOS3.x tables/entrypoint
in follow up patch.

while at it, clear smbios_tables pointer after freeing.
that will avoid double free if smbios_get_tables() is called
multiple times.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Message-Id: <20240314152302.2324164-13-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>