OvmfPkg ResetSystemLib: Fix shutdown via UEFI runtime services

OvmfPkg ResetSystemLib: Fix shutdown via UEFI runtime services

When the PM base address was moved from 0x400 to 0xb000, this
code was missed. This prevented shutdown's via the UEFI system
call from working. (For example, at the EFI shell prompt: reset -s)

We now use gUefiOvmfPkgTokenSpaceGuid.PcdAcpiPmBaseAddress
which is currently set at 0xb000.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14492 6f19259b-4bc3-4df7-8a09-765794883524