qemuBuildMemoryBackendProps: Get pagesize early

qemuBuildMemoryBackendProps: Get pagesize early

https://bugzilla.redhat.com/show_bug.cgi?id=1693066

Up until memfd introduction (in 24b74d187ca) we did not need to
know @pagesize because qemuGetDomainHupageMemPath() could deal
with it being zero (value of zero means use the default hugetlbfs
mount). But since for memfd we are not passing a path to
hugetlbfs mount rather the page size value we need to know its
value upfront.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>