OvmfPkg/PlatformBootManagerLib: Connect the Xen drivers before loading NvVars

OvmfPkg/PlatformBootManagerLib: Connect the Xen drivers before loading NvVars

When OVMF tried to load the file-based NvVars, it checked all the PCI
instances and connected the drivers to the mass storage device. However,
Xen registered its PCI device with a special class id (0xFF80), so
ConnectRecursivelyIfPciMassStorage() couldn't recognize it and skipped the
driver connecting for Xen PCI devices. In the end, the Xen block device
wasn't initialized until EfiBootManagerConnectAll() was called, and it's
already too late to load NvVars.

This commit connects the Xen drivers in ConnectRecursivelyIfPciMassStorage()
so that Xen can use the file-based NvVars.

v3:
* Introduce XenDetected() to cache the result of Xen detection instead
  of relying on PcdPciDisableBusEnumeration.

v2:
* Cosmetic changes

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