hw/smbios: support loading OEM strings values from a file

hw/smbios: support loading OEM strings values from a file

Some applications want to pass quite large values for the OEM strings
entries. Rather than having huge strings on the command line, it would
be better to load them from a file, as supported with -fw_cfg.

This introduces the "path" parameter allowing for:

  $ echo -n "thisthing" > mydata.txt
  $ qemu-system-x86_64 \
    -smbios type=11,value=something \
    -smbios type=11,path=mydata.txt \
    -smbios type=11,value=somemore \
    ...other args...

Now in the guest

$ dmidecode -t 11
Getting SMBIOS data from sysfs.
SMBIOS 2.8 present.

Handle 0x0E00, DMI type 11, 5 bytes
OEM Strings
	String 1: something
	String 2: thisthing
	String 3: somemore

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200923133804.2089190-2-berrange@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>