tests/qtest: Allow qtest_qemu_binary to use a custom environment variable

tests/qtest: Allow qtest_qemu_binary to use a custom environment variable

We're adding support for testing migration using two different QEMU
binaries. We'll provide the second binary in a new environment
variable.

Allow qtest_qemu_binary() to receive the name of the new variable. If
the new environment variable is not set, that's not an error, we use
QTEST_QEMU_BINARY as a fallback.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018192741.25885-2-farosas@suse.de>