make mem_path local variable

It's no longer used anywhere beside main(),
so make it local variable that is used for CLI compat
purposes to keep -mem-path option working.

Under hood QEMU will use it to create
  memory-backend-file,mem-path=...
backend and use its MemoryRegion as main RAM.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200219160953.13771-76-imammedo@redhat.com>

include/sysemu/sysemu.h

@@ -50,7 +50,6 @@ extern uint8_t *boot_splash_filedata;
 extern bool enable_mlock;
 extern bool enable_cpu_pm;
 extern QEMUClockType rtc_clock;
-extern const char *mem_path;
 extern int mem_prealloc;
 
 #define MAX_OPTION_ROMS 16

vl.c

@@ -140,7 +140,6 @@ enum vga_retrace_method vga_retrace_method = VGA_RETRACE_DUMB;
 int display_opengl;
 const char* keyboard_layout = NULL;
 ram_addr_t ram_size;
-const char *mem_path = NULL;
 int mem_prealloc = 0; /* force preallocation of physical target memory */
 bool enable_mlock = false;
 bool enable_cpu_pm = false;
@@ -2883,6 +2882,7 @@ int main(int argc, char **argv, char **envp)
     Error *err = NULL;
     bool list_data_dirs = false;
     char *dir, **dirs;
+    const char *mem_path = NULL;
     BlockdevOptionsQueue bdo_queue = QSIMPLEQ_HEAD_INITIALIZER(bdo_queue);
     QemuPluginList plugin_list = QTAILQ_HEAD_INITIALIZER(plugin_list);