OvmfPkg/OvmfXen: fix build by providing QemuLoadImageLib resolution

OvmfPkg/OvmfXen: fix build by providing QemuLoadImageLib resolution

Commit 859b55443a4253ba ("OvmfPkg/PlatformBootManagerLib: switch to
QemuLoadImageLib") replaced a dependency on LoadLinuxLib with one on
QemuLoadImageLib in the PlatformBootManagerLib implementation that is
shared between all OVMF builds, without taking into account that even
the Xen targeted builds incorporate this code, which is only used to
load kernels passed via the QEMU command line.

Since this is dead code on Xen, we can satisfy the dependency using
the generic version of QemuLoadImageLib, which does not rely on
LoadLinuxLib, which we can therefore drop from OvmfXen.dsc.

Fixes: 859b55443a4253bad8bb618d04a51b2ded67f24b
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>