Fix the cirrus vga model to not leak memory when switching between

Fix the cirrus vga model to not leak memory when switching between
initial vga mode and linear framebuffer.

When creating device model, the vga memory is allocated through
xc_domain_memory_increase_reservation(), however, when switch to use
linear framebuffer, we didn't free that memory, and re-allocate the vga
memory through set_mm_mapping(), that caused memory leak. Now it is
changed to just using qemu_malloc when create device model, and free
that memory when switch to linear framebuffer.

After unset_vram_mapping(), the old memory pointer should be
unmapped; after set_vram_mapping(), the old memory should be freed.

Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>