OvmfPkg/PciHostBridgeLib: silence IA32 VS2015x86 warnings

OvmfPkg/PciHostBridgeLib: silence IA32 VS2015x86 warnings

When compiling "OvmfPkg\Library\PciHostBridgeLib\XenSupport.c" for IA32,
the VS2015x86 compiler emits the following:

> XenSupport.c(41): error C2220: warning treated as error - no 'object'
>                                file generated
> XenSupport.c(41): warning C4244: 'function': conversion from 'UINT64' to
>                                  'UINTN', possible loss of data
> XenSupport.c(48): warning C4244: 'function': conversion from 'UINT64' to
>                                  'UINTN', possible loss of data
> XenSupport.c(49): warning C4244: 'function': conversion from 'UINT64' to
>                                  'UINTN', possible loss of data
> XenSupport.c(50): warning C4244: 'function': conversion from 'UINT64' to
>                                  'UINTN', possible loss of data
> XenSupport.c(222): warning C4244: 'function': conversion from 'UINT64'
>                                   to 'UINTN', possible loss of data
> XenSupport.c(241): warning C4244: 'function': conversion from 'UINT64'
>                                   to 'UINTN', possible loss of data

PciLib functions take UINTN addresses that were encoded with the
PCI_LIB_ADDRESS() macro. We carry addresses from the macro invocations to
the function calls in two UINT64 variables however. This loses no data,
but it alerts VS2015x86. Change the variable types to UINTN.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>