Move initial boot-timer timestamp
Move initial boot-timer timestamp

The initial timestamp passed to the boot-timer to calculate
uVM boot time was taken after creating guest memory, loading the
kernel, and loading the initrd, which should be included
when calculating boot time.

Signed-off-by: Ben Holmes <08e2cb4a1b2c1d0454c46e7f9834fb220ad00ca0@gmail.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker