configure: Allow builds without any system or user emulation

configure: Allow builds without any system or user emulation

The old code aborted configure when no emulation target was selected.
Even after removing the 'exit 1', it tried to read from STDIN
when QEMU was configured with

    configure' '--disable-user' '--disable-system'

This is fixed here.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 8bdd3d499fe0ddffa9901c56ab3bc8911d5b8be0)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>