e1000e: correctly tear down MSI-X memory regions

MSI-X has been disabled by the time the e1000e device is unrealized, hence
msix_uninit is never called.  This causes the object to be leaked, which
shows up as a RAMBlock with empty name when attempting migration.

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>