doc: Fix indefinite article an -> a before nouns that begin with 'h'

Although the rule also allows 'an' for historical reasons,
'a' is more common because 'h' is pronounced.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-doc.texi

@@ -78,7 +78,7 @@ to ease cross-compilation and cross-debugging.
 
 @end itemize
 
-QEMU can run without an host kernel driver and yet gives acceptable
+QEMU can run without a host kernel driver and yet gives acceptable
 performance.
 
 For system emulation, the following hardware targets are supported:

qemu-tech.texi

@@ -536,7 +536,7 @@ timers, especially together with the use of bottom halves (BHs).
 @node Hardware interrupts
 @section Hardware interrupts
 
-In order to be faster, QEMU does not check at every basic block if an
+In order to be faster, QEMU does not check at every basic block if a
 hardware interrupt is pending. Instead, the user must asynchronously
 call a specific function to tell that an interrupt is pending. This
 function resets the chaining of the currently executing basic