hvmloader: Add OVMF UEFI support and directly use it

hvmloader: Add OVMF UEFI support and directly use it
...when specified in the guest configuration file.

This work is somewhat based on Bei Guan effort during the SoC 2011 and
relies on upstream edk2/ovmf Tianocore ROM to be built separately and
manually copied as:
Build/OvmfX64/DEBUG_GCC44/FV/OVMF.fd ->
tools/firmware/ovmf/ovmf-x64.bin
Build/OvmfIa32/DEBUG_GCC44/FV/OVMF.fd ->
toolf/firmware/ovmf/ovmf-ia32.bin

A way to integrate OVMF build directly into XEN has still be discussed
on the mailing list appropriately.

Signed-off-by: Attilio Rao <attilio.rao@citrix.com>
Disable CONFIG_OVMF by default as ovmf is not integrated into the
build.

Signed-off-by: Keir Fraser <keir@xen.org>
Committed-by: Keir Fraser <keir@xen.org>