vfio: Use warn_report() & friends to report warnings

vfio: Use warn_report() & friends to report warnings

The vfio code reports warnings like

    error_report(WARN_PREFIX "Could not frobnicate", DEV-NAME);

where WARN_PREFIX is defined so the message comes out as

    vfio warning: DEV-NAME: Could not frobnicate

This usage predates the introduction of warn_report() & friends in
commit 97f40301f1d.  It's time to convert to that interface.  Since
these functions already prefix the message with "warning: ", replace
WARN_PREFIX by VFIO_MSG_PREFIX, so the messages come out like

    warning: vfio DEV-NAME: Could not frobnicate

The next commit will replace ERR_PREFIX.

Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-6-armbru@redhat.com>