docs/linux: updated kvm config command
docs/linux: updated kvm config command

'make kvmconfig' was replaced with 'make kvm_guest.config' after linux 5.10.

xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller