OvmfPkg: introduce XenMemMapInitialization

OvmfPkg: introduce XenMemMapInitialization

This function parses E820 map provided by Xen and arrange memory maps
accordingly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
[jordan.l.justen@intel.com: XenGetE820Map: VS2010 compat; add assert]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14945 6f19259b-4bc3-4df7-8a09-765794883524