QEMU 5.2.0 configure does not resolve gdbus_codegen correctly

[Kinsey Moore]

When resolving gdbus_codegen from gio-2.0 packageconfig, the resulting binary name (gdbus-codegen) does not include a path. The configure script tests the binary name with -x which fails since that is not the name of a file in the current directory. This causes GDBUS_CODEGEN to be set to empty ("") in QEMU's generated config-host.mak which is consumed by meson which tries to run the present (but empty) contents and fails execution of nothing resulting in a confusing error message and a failure of the build. The patch that will soon be attached forces absolute path resolution of the contents of gdbus_codegen and omits emission of GDBUS_CODEGEN if gdbus_codegen is empty.