vfio-pci: Fix missing unparent of dynamically allocated MemoryRegion

Commit d8d95814609e added explicit object_unparent() calls for
dynamically allocated MemoryRegions.  The VFIOMSIXInfo structure also
contains such a MemoryRegion, covering the mmap'd region of a PCI BAR
above the MSI-X table.  This structure is freed as part of the class
exit function and therefore also needs an explicit object_unparent().
Failing to do this results in random segfaults due to fields within
the structure, often the class pointer, being reclaimed and corrupted
by the time object_finalize_child_property() is called for the object.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org # 2.2
