vl: allow not specifying size in -m when using -M memory-backend

vl: allow not specifying size in -m when using -M memory-backend

Starting in QEMU 6.0's commit f5c9fcb82d ("vl: separate
qemu_create_machine", 2020-12-10), a function have_custom_ram_size()
replaced the return value of set_memory_options().

The purpose of the return value was to record the presence of
"-m size", and if it was not there, change the default RAM
size to the size of the memory backend passed with "-M
memory-backend".

With that commit, however, have_custom_ram_size() is now queried only
after set_memory_options has stored the fixed-up RAM size in QemuOpts for
"future use".  This was actually the only future use of the fixed-up RAM
size, so remove that code and fix the bug.

Cc: qemu-stable@nongnu.org
Fixes: f5c9fcb82d ("vl: separate qemu_create_machine", 2020-12-10)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>