early_p2m_alloc_middle() allocates a new leaf page and
early_p2m_alloc() allocates a new middle page.  This is confusing.

Swap the names so they match what the functions actually do.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>