slirp: tftp: Rework filename handling

This changes the filename handling from a static buffer in tftp_session
for the client-provided name + prefix to a dynamically allocated buffer
that keeps the combined path in one place.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>