KVM: arm64: selftests: Disable single-step without relying on ucall()

Automatically disable single-step when the guest reaches the end of the
verified section instead of using an explicit ucall() to ask userspace to
disable single-step.  An upcoming change to implement a pool-based scheme
for ucall() will add an atomic operation (bit test and set) in the guest
ucall code, and if the compiler generate "old school" atomics, e.g.

  40e57c:       c85f7c20        ldxr    x0, [x1]
  40e580:       aa100011        orr     x17, x0, x16
  40e584:       c80ffc31        stlxr   w15, x17, [x1]
  40e588:       35ffffaf        cbnz    w15, 40e57c <__aarch64_ldset8_sync+0x1c>

the guest will hang as the local exclusive monitor is reset by eret,
i.e. the stlxr will always fail due to the debug exception taken to EL2.

Link: https://lore.kernel.org/all/[email protected]
Cc: Oliver Upton <[email protected]>
Cc: Marc Zyngier <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Oliver Upton <[email protected]>

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