OvmfPkg: PlatformPei: factor out GetFirstNonAddress()

OvmfPkg: PlatformPei: factor out GetFirstNonAddress()

Factor out the expression that is currently the basis of the address width
calculation into a standalone function. In the next patches we'll raise
the return value under certain circumstances.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Thomas Lamprecht <t.lamprecht@proxmox.com>
Ref: https://github.com/tianocore/edk2/issues/59
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>