selftests: kvm: replace numbered sync points with actions

selftests: kvm: replace numbered sync points with actions

Rework the guest=>host syncs in the AMX test to use named actions instead
of arbitrary, incrementing numbers.  The "stage" of the test has no real
meaning, what matters is what action the test wants the host to perform.
The incrementing numbers are somewhat helpful for triaging failures, but
fully debugging failures almost always requires a much deeper dive into
the test (and KVM).

Using named actions not only makes it easier to extend the test without
having to shift all sync point numbers, it makes the code easier to read.

[Commit message by Sean Christopherson]

Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>