We allow either tables or fields for the same type.  Makes sense,
because SeaBIOS uses fields only when no tables are present.

We do this by searching the SMBIOS blob for a previously added table
or field.  Error messages look like this:

    qemu-system-x86_64: -smbios type=1,serial=42: SMBIOS type 1 table already defined, cannot add field

User needs to know that "table" is defined by -smbios file=..., and
"field" by -smbios type=...

Instead of searching the blob, record additions of interest, and check
that.  Simpler, and makes better error messages possible:

    qemu-system-x86_64: -smbios file=smbios_type_1.bin: Can't mix file= and type= for same type
    qemu-system-x86_64: -smbios type=1,serial=42,serial=99: This is the conflicting setting

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>