selftests: KVM: Add helpers for vCPU device attributes

vCPU file descriptors are abstracted away from test code in KVM
selftests, meaning that tests cannot directly access a vCPU's device
attributes. Add helpers that tests can use to get at vCPU device
attributes.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210916181555.973085-5-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>