machine: add mem compound property

Make -m syntactic sugar for a compound property "-machine
mem.{size,max-size,slots}".  The new property does not have
the magic conversion to megabytes of unsuffixed arguments,
and also does not understand that "0" means the default size
(you have to leave it out to get the default).  This means
that we need to convert the QemuOpts by hand to a QDict.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414165300.555321-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>