OvmfPkg: QemuVideoDxe: tidy up error checking/handling in & under Start()

OvmfPkg: QemuVideoDxe: tidy up error checking/handling in & under Start()

In QemuVideoControllerDriverStart():
- remove redundant zero-initialization of:
  - Private->Handle (2 locations)
  - Private->GopDevicePath (when at devpath end)

- remove fields used for error handling only:
  - PciAttributesSaved

- tigthen scope of temporaries:
  - MmioDesc
  - AcpiDeviceNode

- supplement missing error checks:
  - AppendDevicePathNode() can fail with out-of-memory (2 locations)
  - when installing GopDevicePath
  - retval of QemuVideoGraphicsOutputConstructor() (can justifiedly fail
    with out-of-resources)

- plug leaks on error:
  - free GopDevicePath (AppendDevicePathNode() allocates dynamically)
  - uninstall GopDevicePath
  - free Private->ModeData
  - call QemuVideoGraphicsOutputDestructor()
  - uninstall GOP

In QemuVideoGraphicsOutputConstructor(), called by Start():
- supplement missing error checks:
  - QemuVideoGraphicsOutputSetMode() retval (it can fail with
    out-of-resources)

- plug leaks on error:
  - free Mode->Info
  - free Mode

In QemuVideoCirrusModeSetup() and QemuVideoBochsModeSetup(), both called
by Start():
- supplement missing error checks:
  - AllocatePool() can fail in both

In QemuVideoGraphicsOutputDestructor(), called by Start() on the error
path:
- plug leaks:
  - free Private->LineBuffer, which is allocated in
    Start() -> Constructor() -> SetMode()

In QemuVideoGraphicsOutputSetMode(), called by Start() indirectly:
- remove redundant zero-assignment to:
  - Private->LineBuffer

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@15282 6f19259b-4bc3-4df7-8a09-765794883524