x86: make the pv-only e820 array be dynamic.

x86: make the pv-only e820 array be dynamic.

During creation of the PV domain we allocate the E820 structure to
have the amount of E820 entries on the machine, plus the number three.

This will allow the tool stack to fill the E820 with more than three
entries. Specifically the use cases is , where the toolstack retrieves
the E820, sanitizes it, and then sets it for the PV guest (for PCI
passthrough), this dynamic number of E820 is just right.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Keir Fraser <keir@xen.org>