vmm/lib.rs: no more console=ttyS0 by default
vmm/lib.rs: no more console=ttyS0 by default

Removed "console=ttyS0" from the default kernel command line. This
reduces the number of VM exits caused by reads/writes to the serial
port, which shaves off tens of ms from the effective boot time. We
also add the "8250.nr_uarts=0" option, which skips even more serial
port related initialization time.

Signed-off-by: Alexandru Agache <69826f943dd6ae0c808126caf67f094b6aee96c2@amazon.com>

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