OvmfPkg/XenBusDxe: Open PciIo protocol.

OvmfPkg/XenBusDxe: Open PciIo protocol.

The PciIo interface will be used in "OvmfPkg/XenBusDxe: Add Grant Table
functions" to get the memory address of the BAR 1 and use the space to
map shared memory.

Change in V3:
- add a commit description.

Change in V2:
- Coding style
- Error handler

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16261 6f19259b-4bc3-4df7-8a09-765794883524