pcie: Don't try triggering a LSI when not defined

pcie: Don't try triggering a LSI when not defined

This patch skips [de]asserting a LSI interrupt if the device doesn't
have any LSI defined. Doing so would trigger an assert in
pci_irq_handler().

The PCIE root port implementation in qemu requests a LSI (INTA), but a
subclass may want to change that behavior since it's a valid
configuration. For example on the POWER8/POWER9/POWER10 systems, the
root bridge doesn't request any LSI.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220408131303.147840-2-fbarrat@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>