qemu-log: support simple pid substitution for logs

qemu-log: support simple pid substitution for logs

When debugging stuff that occurs over several forks it would be useful
not to keep overwriting the one logfile you've set-up. This allows a
simple %d to be included once in the logfile parameter which is
substituted with getpid().

As the test cases involve checking user output they need
g_test_trap_subprocess() support. As a result they are currently skipped
on Travis builds due to the older glib involved.

Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Leandro Dorileo <[email protected]>
Reviewed-by: Aurelien Jarno <[email protected]>
Reviewed-by: Richard Henderson  <[email protected]>
Message-Id: <1458052224[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>