trace: support tracepoint signatures of arbitrary length

Currently tracepoint's signature string is encoded into u64 which
gives 8 character limit to the signature. When signature does not fit
into that limit, only the first 8 characters are preserved.

This patch fixes the problem by storing the signature as a C string of
arbitrary length.

Fixes #288.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>