libxl: Add support for passing in the host's E820 for PCI passthrough

libxl: Add support for passing in the host's E820 for PCI passthrough

The code that populates E820 is unconditionally triggered by the guest
configuration having "pci=['<BDF>,..']", being a PV guest, and if
b_info->u.pv.e820_host is set.

The code do_domain_create calls the libxl__e820_alloc when
it notices that the guest is PV, has at least one PCI devices, and has
the e820_host flag set.

libxl__e820_alloc calls the xc_get_machine_memory_map to retrieve the systems
E820. Then the E820 is sanitized to weed out E820 entries below 16MB, and as
well remove any E820_RAM or E820_UNUSED regions as the guest does not need to
know about them. The guest only needs the E820_ACPI, E820_NVS, E820_RESERVED to
get an idea of where the PCI I/O space is. Mostly.. The Linux kernel assumes that any
gap in the E820 is considered PCI I/O space which means that if we pass
in the guest 2GB, and the E820_ACPI, and its friend start at 3GB, the
gap between 2GB and 3GB will be considered as PCI I/O space. To guard against
that we also create an E820_UNUSABLE between the region of 'target_kb'
(called ram_end in the code) up to the first E820_[ACPI,NVS,RESERVED] region.
Lastly, the xc_domain_set_memory_map is called to install the new E820.

When tested with another PV guest (NetBSD 5.1) the modified E820 gave
it no trouble. The code has also been tested with older "classic" Xen Linux
and with the newer "pvops" with success (SLES11, RHEL5, Ubuntu Lucid,
Debian Squeeze, 2.6.37, 2.6.38, 2.6.39).

Memory that is slack or for balloon (so 'maxmem' in guest configuration)
is put behind the machine E820. Which in most cases is after the 4GB.

The reason for doing the fetching of the E820 using the hypercall in
the toolstack (instead of the guest doing it) is that when a guest
would do a hypercall to 'XENMEM_machine_memory_map' it would
retrieve an E820 with I/O range caps added in. Meaning that the
region after 4GB up to end of possible memory would be marked as unusable
and the kernel would not have any space to allocate a balloon
region.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>