xen/x86: actually allocate legacy interrupts on PV guests

b4ff8389 is incomplete: relies on nr_legacy_irqs() to get the number
of legacy interrupts when actually nr_legacy_irqs() returns 0 after
probe_8259A(). Use NR_IRQS_LEGACY instead.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
CC: stable@vger.kernel.org