builtin-run: Move kernel command line generation to a separate function

The real kernel command line is gradually generated in kvm_cmd_run_init()
and it is interspersed with the initialization code. This means that both
the code that generates the command line and the rest of the code is
unnecessarily difficult to follow and to modify. Move the code that
generates the command line to one function, to make it easier to
understand, and to declutter kvm_cmd_run_init().

No functional change intended.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20210923144505.60776-7-alexandru.elisei@arm.com


Signed-off-by: Will Deacon <will@kernel.org>