libxl: Only map legacy PCI IRQs if they are supported

libxl: Only map legacy PCI IRQs if they are supported

Arm's PCI passthrough implementation doesn't support legacy interrupts,
but MSI/MSI-X. This can be the case for other platforms too.
For that reason introduce a new CONFIG_PCI_SUPP_LEGACY_IRQ and add
it to the CFLAGS and compile the relevant code in the toolstack only if
applicable.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[stefano: minor change to Makefile]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Tested-by: Rahul Singh <rahul.singh@arm.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>