qemuBuildMemoryBackendProps: Use boolean type for 'pmem' property

qemuBuildMemoryBackendProps: Use boolean type for 'pmem' property

Commit 82576d8f35e used a string "on" to enable the 'pmem' property.
This is okay for the command line visitor, but the property is declared
as boolean in qemu and thus it will not work when using QMP.

Modify the type to boolean. This changes the command line, but
fortunately the command line visitor in qemu parses both 'yes' and 'on'
as true for the property.

https://bugzilla.redhat.com/show_bug.cgi?id=1854684

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>