hostmem: Don't report pmem attribute if unsupported

hostmem: Don't report pmem attribute if unsupported

When management applications (like Libvirt) want to check whether
memory-backend-file.pmem is supported they can list object
properties using 'qom-list-properties'. However, 'pmem' is
declared always (and thus reported always) and only at runtime
QEMU errors out if it was built without libpmem (and thus can not
guarantee write persistence). This is suboptimal since we have
ability to declare attributes at compile time.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1915216
Message-Id: <dfcc5dc7e2efc0283bc38e3036da2c0323621cdb.1611647111.git.mprivozn@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>