vm/qemu: support 9p host-based image
vm/qemu: support 9p host-based image

If "image" is set to "9p" in config file,
qemu VM will create a minimalistic image based
on readonly-mapped host filesystem.
The main things that we need are working sshd and ssh-keygen.
/tmp, /etc/, /var, /root are remounted as tmpfs.

zoulasc/syzkaller,xairy/syzkaller,xairy/syzkaller,dotweiba/syzkaller,xairy/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,zoulasc/syzkaller,google/syzkaller,sploving/syzkaller,sploving/syzkaller,dotweiba/syzkaller,sploving/syzkaller,hyz0906/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,zoulasc/syzkaller,hyz0906/syzkaller,google/syzkaller,sploving/syzkaller,xairy/syzkaller,hyz0906/syzkaller,xairy/syzkaller,hyz0906/syzkaller,xairy/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller