machine: add default_ram_size to machine class

Machines types can have different requirement for default ram
size. Introduce a member in the machine class and set the current
default_ram_size to 128MB.

For QEMUMachine types override the value during the registration of
the machine and for MachineClass introduce the generic class init
setting the default_ram_size.

Add helpers [K,M,G,T,P,E]_BYTE for better readability and easy usage

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
