xl, e820_host, PV passthrough: Fix guests crashing when memory == maxmem

xl, e820_host, PV passthrough: Fix guests crashing when memory == maxmem

The code had an obvious bug where it would assume that the balloon
amount would always be _something_ and add an E820_RAM entry at the
end of the E820 array. The added E820_RAM would contain the balloon amount
plus the delta of memory that had to be subtracted b/c of the various
E820 entries. That assumption is certainly true when maxmem != mem,
but if guest config has maxmem = memory that is incorrect (as balloon
value is zero). The end result is that the E820 that is constructed
is missing a swath of "delta" memory and in most cases ends up with
only one E820_RAM entry that is of 512MB size on many Intel systems.

Reported-by: Christian Holpert <christian@holpert.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>