OvmfPkg/OvmfXen: Introduce DEBUG_ON_HYPERVISOR_CONSOLE build flag

Introduce DEBUG_ON_HYPERVISOR_CONSOLE build flag to enable logging
debug output to the Xen console.

This will work with both Xen HVM guest and Xen PVH guest whereas the
default PlatformDebugLibIoPort works only in HVM when QEMU is present.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200423095358.2518197-6-anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>