block: Move common QMP commands to block-core QAPI module

block-core is for everything that isn't related to the system emulator.
Internal snapshots, the NBD server and quorum events make sense in the
tools, too, so move them to block-core.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-5-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>