KVM: selftests: Fix dynamic generation of configuration names

When we dynamically generate a name for a configuration in get-reg-list
we use strcat() to append to a buffer allocated using malloc() but we
never initialise that buffer. Since malloc() offers no guarantees
regarding the contents of the memory it returns this can lead to us
corrupting, and likely overflowing, the buffer:

  vregs: PASS
  vregs+pmu: PASS
  sve: PASS
  sve+pmu: PASS
  vregs+pauth_address+pauth_generic: PASS
  X?vr+gspauth_addre+spauth_generi+pmu: PASS

The bug is that strcat() should have been strcpy(), and that replacement
would be enough to fix it, but there are other things in the function
that leave something to be desired.  In particular, an (incorrectly)
empty config would cause an out of bounds access to c->name[-1].
Since the strcpy() call relies on c->name[0..len-1] being initialized,
enforce that invariant throughout the function.

Fixes: 2f9ace5d ("KVM: arm64: selftests: get-reg-list: Introduce vcpu configs")
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Co-developed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Message-Id: <20231211-kvm-get-reg-list-str-init-v3-1-6554c71c77b1@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>