Re-enables ptrace for paravirtualized guests.

Re-enables ptrace for paravirtualized guests.

Changes:

- Added to_ma function which translates a physical address to a
  machine address for HVM guests and just returns the address for PV
  guests.

- Use to_ma() in map_domain_va_32, map_domain_va_pae and
  map_domain_va_64 to abstract away differences between HVM and PV
  guests.

- (From Keir, Ryan) Export CR0 to PV guests as well.

I've removed the address checks against 0 since the last version since
physical/machine address 0 should be valid. Invalid translations
should be catched in xc_map_foreign_domain.

Signed-Off-By: Simon Kagstrom <simon.kagstrom@bth.se>