xen/balloon: Set balloon's initial state to number of existing RAM pages

Currently balloon's initial value is set to max_pfn which includes
non-RAM ranges such as MMIO hole. As result, initial memory target
(specified by guest's configuration file) will appear smaller than
what balloon driver perceives to be the current number of available
pages. Thus it will balloon down "extra" pages, decreasing amount of
available memory for no good reason.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>