block: Use warn_report() & friends to report warnings

block: Use warn_report() & friends to report warnings

Calling error_report() in a function that takes an Error ** argument
is suspicious.  Convert a few that are actually warnings to
warn_report().

While there, split warnings consisting of multiple sentences to
conform to conventions spelled out in warn_report()'s contract, and
improve a rather useless warning in sheepdog.c.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Lieven <pl@kamp.de>
Cc: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-4-armbru@redhat.com>

Drop changes to "without an explicit read-only=on" warnings, because
there's a series removing them pending.  Also drop a cc: to a former
Sheepdog maintainer.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>