Add one new file vga-pci.h and cleanup on all platforms

Functions pci_vga_init() and pci_cirrus_vga_init() are declared
in pc.h. That prevents other platforms (e.g. sPAPR) to use them.

This patch is to create one new file vga-pci.h and move the
declarations to vga-pci.h, so that they can be shared by
all platforms. This patch also cleans up on all platforms.

Signed-off-by: Li Zhang <zhlcindy@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
