xen/setup: Ignore E820_UNUSABLE when setting 1-1 mappings.

When we parse the raw E820, the Xen hypervisor can set "E820_RAM"
to "E820_UNUSABLE" if the mem=X argument is used. As such we
should _not_ consider the E820_UNUSABLE as an 1-1 identity
mapping, but instead use the same case as for E820_RAM.

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