kvm: set gsi_bits and max_gsi correctly

The current kvm_init_irq_routing() doesn't set up the used_gsi_bitmap
correctly, and as a consequence pins max_gsi to 32 when it really
should be 1024. I ran into this limitation while testing pci
passthrough, where I consistently got an -ENOSPC return from
kvm_get_irq_route_gsi() called from assigned_dev_update_msix_mmio().

Signed-off-by: Jason Baron <jbaron@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
