xen/p2m: Fix the comment describing the P2M tree.

It mixed up the p2m_mid_missing with p2m_missing. Also
remove some extra spaces.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>