qapi: Clean up cgen() and mcgen()

Commit 05dfb26 added eatspace stripping to mcgen(). Move it to
cgen(), just in case somebody gets tempted to use cgen() directly
instead of via mcgen().

cgen() indents blank lines. No such lines get generated right now,
but fix it anyway.

We use triple-quoted strings for program text, like this:

'''
Program text
any number of lines
'''

Keeps the program text relatively readable, but puts an extra newline
at either end. mcgen() "fixes" that by dropping the first and last
line outright. Drop only the newlines.

This unmasks a bug in qapi-commands.py: four quotes instead of three.
Fix it up.

Output doesn't change

Backports commit 77e703b861d34bb2879f3e845482d5cf0a3a0ad1 from qemu

qemu/scripts/qapi.py

@@ -942,15 +942,20 @@ def pop_indent(indent_amount=4):
     global indent_level
     indent_level -= indent_amount
 
+# Generate @code with @kwds interpolated.
+# Obey indent_level, and strip eatspace.
 def cgen(code, **kwds):
-    indent = genindent(indent_level)
-    lines = code.split('\n')
-    lines = map(lambda x: indent + x, lines)
-    return '\n'.join(lines) % kwds + '\n'
+    raw = code % kwds
+    if indent_level:
+        indent = genindent(indent_level)
+        raw = re.subn("^.", indent + r'\g<0>', raw, 0, re.MULTILINE)
+        raw = raw[0]
+    return re.sub(re.escape(eatspace) + ' *', '', raw)
 
 def mcgen(code, **kwds):
-    raw = cgen('\n'.join(code.split('\n')[1:-1]), **kwds)
-    return re.sub(re.escape(eatspace) + ' *', '', raw)
+    if code[0] == '\n':
+        code = code[1:]
+    return cgen(code, **kwds)
 
 def basename(filename):
     return filename.split("/")[-1]