smbios: Improve diagnostics for conflicting entries

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>
