conf: Rework <tpm/> formatting

conf: Rework <tpm/> formatting

The <tpm/> element formatting is handled in
virDomainTPMDefFormat() which uses the "old style" - appending
strings directly into the output buffer. With this, it's easy to
get conditions that tell when an element has ended wrong. In this
particular case, if both <encryption/> and <active_pcr_banks/>
are to be formatted the current code puts a stray '>' into the
output buffer, resulting in invalid XML.

Rewrite the function to use virXMLFormatElement() which is more
clever.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2016599#c15
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>