Use vpath directive

Use vpath directive

The vpath directive has two advantages over the VPATH variable:
1) it allows to skip searching of .o files; 2) the default semantics
are to append to the vpath, so there is no confusion between "VPATH=xyz"
and "VPATH+=xyz".

Since "vpath %.c %.h PATH" is not valid, I'm introducing a wrapper
macro to append one or more directories to the vpath.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>