libxl: bump LIBXL_MAXMEM_CONSTANT to 2048

When using OVMF we need to have 1MiB of memory in place for firmware. Without this change we have:

(XEN) HVM128: Loading OVMF ...
(XEN) page_alloc.c:1460:d128 Over-allocation for domain 128: 33025 > 33024
(XEN) memory.c:132:d128 Could not allocate order=0 extent: id=128 memflags=0 (0 of 1)

This is not a fatal error as hvmloader will instead use low memory to
load OVMF, but it's better to eliminate such error.

Changing this constant doesn't necessary increase the total amount of
memory a guest uses because it's just a limit.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Config.mk: update OVMF changeset

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
tools: clone ovmf to ovmf-dir directory

For consistency with other foo-dir e.g. qemu, seabios.

Remove obsolete ovmf-find target.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
tools: support system supplied ovmf binary

Debian Jessie at least contains an ovmf package that includes
/usr/share/ovmf/OVMF.fd. It's also possible that user may want to supply
his/her own ovmf binary.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
tools: Enable OVMF build by default

The issues with non-GCC4.4 builds seem to have been resolved.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>