strfd: introduce strvprintf() API

variant of strprintf() which accepts va_list as parameter
instead of variable args (...)

Change-Id: I205acf3aaf5440890d5f9cad693c6a358a5d9218
BUG: 1089216
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/7539
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>