xen/x86: actually allocate legacy interrupts on PV guests

[ Upstream commit 702f926067d2a4b28c10a3c41a1172dd62d9e735 ]

b4ff8389ed14 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
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>