OvmfPkg: QemuVideoDxe: disentangle UEFI driver model use in Stop()

OvmfPkg: QemuVideoDxe: disentangle UEFI driver model use in Stop()

A bus driver needs to pay attention whether its Stop() function is being
called on the "main" controller handle (NumberOfChildren == 0) or on the
child handles (NumberOfChildren > 0).

In QemuVideoDxe, all our resources are associated with the one child
handle (and the Private data structure) *except* the top-level PciIo
protocol reference. Be conscious of which mode Stop() is being called for.

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@15284 6f19259b-4bc3-4df7-8a09-765794883524