OvmfPkg: QemuVideoDxe: serialize Start() against callbacks

OvmfPkg: QemuVideoDxe: serialize Start() against callbacks

If Start() succeeds, the callback is only executed when the setup is
complete (on the stack of RestoreTPL()), rather than on the stack of
InstallMultipleProtocolInterfaces(), when the driver setup may yet be
theoretically incomplete.

If Start() fails, the protocol interface will have been uninstalled
(rolled back) by the time the callback runs (again, on the stack of
RestoreTPL()). Since protocol notification callbacks begin with locating
the protocol interface in question, such attempts to locate will fail
immediately and save some work in the callback.

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@15371 6f19259b-4bc3-4df7-8a09-765794883524