[int13con] Preserve all registers across real-mode calls

The BIOS on some older AWS instance types (observed with c3.large in
eu-central-1) does not correctly preserve the upper halves of the
32-bit general-purpose registers.  In particular, calling INT13,42 or
INT13,43 will zero the high half of %ebx.  If the compiled code has a
live value in %ebx then this will result in undefined behaviour.

The defect appears to be AWS-specific: it does not exist in the
upstream Bochs BIOS code from which this BIOS is derived.

The iPXE disk log console is not performance-sensitive but must be as
reliable as possible, since it exists to be the debug log mechanism of
last resort.  We therefore choose the path of maximum paranoia: we
work around this and other potential BIOS bugs by preserving all
non-output registers across all real-mode calls.

Signed-off-by: Michael Brown <mcb30@ipxe.org>