qemu: Adapt to new log format

qemu: Adapt to new log format

Since 586502189edf9fd0f89a83de96717a2ea826fdb0 qemu commit, the log
lines reporting chardev's path has changed from:

$ ./x86_64-softmmu/qemu-system-x86_64 -serial pty -serial pty -monitor pty
char device redirected to /dev/pts/5
char device redirected to /dev/pts/6
char device redirected to /dev/pts/7

to:

$ ./x86_64-softmmu/qemu-system-x86_64 -serial pty -serial pty -monitor pty
char device compat_monitor0 redirected to /dev/pts/5
char device serial0 redirected to /dev/pts/6
char device serial1 redirected to /dev/pts/7

However, with current code we are not prepared for such change, which
results in us being unable to start any domain.