meson: qga: do not use deprecated meson.build_root()

meson: qga: do not use deprecated meson.build_root()

The function will return the build root of the parent project if called from a
subproject; that is irrelevant for QEMU's usage but rarely desirable, and
therefore the function was deprecated and replaced by two functions
project_build_root() and global_build_root().  Replace it with the former.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>