hw/block/nvme: support the get/set features select and save fields

hw/block/nvme: support the get/set features select and save fields

Since the device does not have any persistent state storage, no
features are "saveable" and setting the Save (SV) field in any Set
Features command will result in a Feature Identifier Not Saveable status
code.

Similarly, if the Select (SEL) field is set to request saved values, the
devices will (as it should) return the default values instead.

Since this also introduces "Supported Capabilities", the nsid field is
now also checked for validity wrt. the feature being get/set'ed.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20200706061303.246057-13-its@irrelevant.dk>