OvmfPkg: include XHCI driver

OvmfPkg: include XHCI driver

QEMU commit aa685789 ("xhci: generate a Transfer Event for each Transfer
TRB with the IOC bit set") fixed an emulation problem in QEMU; we can now
drive that host controller with edk2's XhciDxe. Include it in OvmfPkg, as
XHCI emulation is reportedly more virtualization-friendly than EHCI,
consuming less CPU.

The driver can be tested with the following QEMU command line options:

  -device nec-usb-xhci -device usb-kbd

This patch should not regress existing QEMU command lines (ie. trigger an
ASSERT() in XhciDxe that fails on pre-aa685789 QEMU) because QEMU's
"-device nec-usb-xhci" has never before resulted in USB devices that
worked with edk2 firmware builds, hence users have never had a reason to
add that option.

Now that they learn about XHCI support in OVMF by reading this commit
message, they (or their packagers) will also know to update qemu to
aa685789 or later (in practice that means the upcoming 2.3 release), at
least if they want to use '-device nec-usb-xhci' with edk2, for the first
time ever.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Alexander Graf <agraf@suse.de>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17055 6f19259b-4bc3-4df7-8a09-765794883524