qemuBuildMemoryBackendProps: Move @prealloc setting to backend agnostic part

qemuBuildMemoryBackendProps: Move @prealloc setting to backend agnostic part

All three memory backends (-file, -ram and -memfd) have .prealloc
attribute. Since we are setting it only for -file, the
corresponding code lives only under if() that handles that
specific backend. But in near future we will want to set the
attribute for other backends too. Therefore, move the
corresponding code outside of the if().

This causes some .argv files to be changed, but the only change
happening there is move of the attribute (best viewed with:
'git show --color-words=.').

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>