qapi: remove qapi/qapi-types-core.h

The file is only including error.h and qerror.h. Prefer explicit
inclusion of whatever files are needed.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qapi: allow sharing enum implementation across visitors

Most visitors will use the same code for enum parsing. Move it to
the core.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>