OvmfPkg: QemuFlashFvbServicesRuntimeDxe: fix out-of-LBA write access

When QemuFlashWrite() is asked to write a range that includes the last
byte of the LBA, then the byte that the function uses to switch the flash
device back to read mode (ROMD mode in KVM speak) actually falls out of
the LBA.

Normally this doesn't cause visible problems. However, if the variable
store and the firmware code are backed by separate flash devices, as
implemented by

  [Qemu-devel] [PATCH v2] hw/i386/pc_sysfw: support two flash drives
  http://thread.gmane.org/gmane.comp.emulators.qemu/243678

plus

  [edk2] [edk2 PATCH] OvmfPkg: split the variable store to a separate file
  http://thread.gmane.org/gmane.comp.bios.tianocore.devel/5045/focus=5046

then the READ_ARRAY_CMD not only reaches a different LBA, it reaches a
different qemu device. This results in a guest reboot soon after.

Fix this by ensuring that we always stay within the LBA just written when
issuing READ_ARRAY_CMD.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14996 6f19259b-4bc3-4df7-8a09-765794883524