x86/ept: pass correct level to p2m_entry_modify

x86/ept: pass correct level to p2m_entry_modify

EPT differs from NPT and shadow when translating page orders to levels
in the physmap page tables. EPT page tables level for order 0 pages is
0, while NPT and shadow instead use 1, ie: EPT page tables levels
starts at 0 while NPT and shadow starts at 1.

Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
one to the level, in order to match NPT and shadow usage.

While there also add a check to ensure p2m_entry_modify is never
called with level == 0. That should allow to catch future errors
related to the level parameter.

Fixes: c7a4c088ad1c ('x86/mm: split p2m ioreq server pages special handling into helper')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>