char: remove fixed length filename allocation

A variety of places were snprintf()ing into a fixed length
filename buffer. Some of the buffers were stack allocated,
while another was heap allocated with g_malloc(). Switch
them all to heap allocated using g_strdup_printf() avoiding
arbitrary length restrictions.

This also facilitates later patches which will want to
populate the filename by calling external functions
which do not support use of a pre-allocated buffer.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1453202071-10289-2-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>