emulator: create qemu images for system,vendor

For raw system.img and vendor.img do not have
partition table and init refuses to mount them
in first stage mounting.

For emulator build:
create partitioned system-qemu.img for system.img;
create partitioned vendor-qemu.img for vendor.img
if applicable.

Change-Id: I64fdcdafc303cf2f74a1e96653e7afe259f0af98