KVM: selftests: Provide a global pseudo-RNG instance for all tests

Add a global guest_random_state instance, i.e. a pseudo-RNG, so that an
RNG is available for *all* tests.  This will allow randomizing behavior
in core library code, e.g. x86 will utilize the pRNG to conditionally
force emulation of writes from within common guest code.

To allow for deterministic runs, and to be compatible with existing tests,
allow tests to override the seed used to initialize the pRNG.

Note, the seed *must* be overwritten before a VM is created in order for
the seed to take effect, though it's perfectly fine for a test to
initialize multiple VMs with different seeds.

And as evidenced by memstress_guest_code(), it's also a-ok to instantiate
more RNGs using the global seed (or a modified version of it).  The goal
of the global RNG is purely to ensure that _a_ source of random numbers is
available, it doesn't have to be the _only_ RNG.

Link: https://lore.kernel.org/r/20240314185459.2439072-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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