hw/i386/pc: Map into memory the initrd

In order to reduce the memory footprint we map into memory
the initrd using g_mapped_file_new() instead of reading it.
In this way we can share the initrd pages between multiple
instances of QEMU.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190724143105.307042-4-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>