OvmfPkg/Virtio10Dxe: convert to PciCapLib

OvmfPkg/Virtio10Dxe: convert to PciCapLib

Replace the manual capability list parsing in OvmfPkg/Virtio10Dxe with
PciCapLib and PciCapPciIoLib API calls.

The VIRTIO_PCI_CAP_LINK structure type is now superfluous. (Well, it
always has been; we should have used EFI_PCI_CAPABILITY_HDR.)

Also, EFI_PCI_CAPABILITY_VENDOR_HDR is now included at the front of
VIRTIO_PCI_CAP. No driver other than Virtio10Dxe relies on VIRTIO_PCI_CAP.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>