hostmem: introduce "prealloc-threads" property

the property will allow user to specify number of threads to use
in pre-allocation stage. It also will allow to reduce implicit
hostmem dependency on current_machine.
On object creation it will default to 1, but via machine
compat property it will be updated to MachineState::smp::cpus
to keep current behavior for hostmem and main RAM (which is
now also hostmem based).

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200219160953.13771-77-imammedo@redhat.com>