pci-assign: Fix a bug when map MSI-X table memory failed

pci-assign: Fix a bug when map MSI-X table memory failed

When mmapping memory for the MSI-X table failsthe dev->msix_table is
not set to NULL and assigned_dev_unregister_msix_mmio() will cause
a segfault when trying to munmap it.

Signed-off-by: Gonglei Arei <arei.gonglei@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>