bios: Fix multiple calls into smbios_load_ex

We're marking the used entry bitmap in smbios_load_external() for each
type we check, regardless of whether we loaded anything.  This makes
subsequent calls behave as if we've already loaded the tables from qemu
and can result in missing tables (ex. multiple type4 entries on an SMP
guest).  Only mark the bitmap if we actually load something.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
