selftests: kvm: Clear uc so UCALL_NONE is being properly reported

Ensure the out value 'uc' in get_ucall() is properly reporting
UCALL_NONE if the call fails.  The return value will be correctly
reported, however, the out parameter 'uc' will not be.  Clear the struct
to ensure the correct value is being reported in the out parameter.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Alexander Graf <graf@amazon.com>
Message-Id: <20201012194716.3950330-3-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>