pc: leave more space for BIOS allocations

pc: leave more space for BIOS allocations

Since QEMU 2.1, we are allocating more space for ACPI tables, so no
space is left after initrd for the BIOS to allocate memory.

Besides ACPI tables, there are a few other uses of high memory in
SeaBIOS: SMBIOS tables and USB drivers use it in particular.  These uses
allocate a very small amount of memory.  Malloc metadata also lives
there.  So we need _some_ extra padding there to avoid initrd breakage,
but not much.

John Snow found a case where RHEL5 was broken by the recent change to
ACPI_TABLE_SIZE; in his case 4KB of extra padding are fine, but just to
be safe I am adding 32KB, which is roughly the same amount of padding
that was left by QEMU 2.0 and earlier.

Move initrd to leave some space for the BIOS.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: John Snow <jsnow@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>