vubr_set_host() passes char values to isdigit().  Undefined behavior
when the value is negative.

Fix by using qemu_isdigit() instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190514180311.16028-3-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[Missing #include "qemu-common.h" fixed]