qemu_tpm, security, tests: change 'switch' clauses for 'if'

qemu_tpm, security, tests: change 'switch' clauses for 'if'

This trivial rework is aimed to reduce the amount of line changes
made by the next patch, when 'def->tpm' will become a 'def->tpms'
array.

Instead of using a 'switch' where only the VIR_DOMAIN_TPM_TYPE_EMULATOR
label does something, use an 'if' clause instead.

Tested-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>