configure: add option for virtiofsd

Currently it is unknown whether virtiofsd will be built at
configuration time. It will be automatically built when dependency
is met. Also, required libraries are not clear.

To make this clear, add configure option --{enable,disable}-virtiofsd.
The default is the same as current (enabled if available) like many
other options. When --enable-virtiofsd is given and dependency is not
met, we get:

  ERROR: Problem encountered: virtiofsd requires libcap-ng-devel and seccomp-devel
or
  ERROR: Problem encountered: virtiofsd needs tools and vhost-user support

In addition, configuration summary now includes virtiofsd entry:

  build virtiofs daemon: YES/NO

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Message-Id: <20201008103133.2722903-1-misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Manual merge