Add Error argument for pci_add_capability() to leverage the errp
to pass info on errors. This way is helpful for its callers to
make a better error handling when moving to 'realize'.

Cc: pbonzini@redhat.com
Cc: rth@twiddle.net
Cc: ehabkost@redhat.com
Cc: mst@redhat.com
Cc: jasowang@redhat.com
Cc: marcel@redhat.com
Cc: alex.williamson@redhat.com
Cc: armbru@redhat.com
Signed-off-by: NMao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>