memory: reorganize file-based allocation

Split the internal part to a separate function.  The next patch
will also split the external parts.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
memory: move mem_path handling to memory_region_allocate_system_memory

Other RAM regions will now be backed by regular RAM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
memory: add file-based HostMemoryBackend

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
zynq: exit if too much RAM is specified

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
virtex_ml507: remove useless arguments

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
vexpress: pass QEMUMachineInitArgs to VEDBoardInfo init function

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
boards: add comment where we cannot use memory_region_allocate_system_memory

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
boards: use memory_region_allocate_system_memory

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>